NuttX: Add missing include header in IRQ implementation

[gustavonihei]

This PR intends to add a missing header to the irq.c file, which became evident after a code refactor in another component and resulted in the following build error:

libmetal/lib/system/nuttx/irq.c: In function ‘metal_cntr_irq_set_enable’:                                                                                                                                                                                                                 
libmetal/lib/system/nuttx/irq.c:34:25: error: implicit declaration of function ‘up_enable_irq’ [-Werror=implicit-function-declaration]                                                                                                                                                    
   34 |                         up_enable_irq(irq);                                                                                                                                                                                                                                       
      |                         ^~~~~~~~~~~~~                                                                                                                                                                                                                                             
libmetal/lib/system/nuttx/irq.c:36:25: error: implicit declaration of function ‘up_disable_irq’ [-Werror=implicit-function-declaration]                                                                                                                                                   
   36 |                         up_disable_irq(irq);                                                                                                                                                                                                                                      
      |                         ^~~~~~~~~~~~~~                                                                                                                                                                                                                                            
libmetal/lib/system/nuttx/irq.c: In function ‘metal_cntr_irq_attach’:                                                                                                                                                                                                                     
libmetal/lib/system/nuttx/irq.c:73:17: error: implicit declaration of function ‘irq_dispatch’; did you mean ‘irq_detach’? [-Werror=implicit-function-declaration]                                                                                                                         
   73 |                 irq_dispatch(irq, NULL); /* fake a irq request */                                                                                                                                                                                                                 
      |                 ^~~~~~~~~~~~                                                                                                                                                                                                                                                      
      |                 irq_detach                                                                                                                                                                                                                                                        
cc1: all warnings being treated as errors                                                                                                                                                                                                                                                 
make[1]: *** [Makefile:43: libmetal/lib/system/nuttx/irq.o] Error 1