Cast pthread_self() as uintptr_t

[Dennis14e]

This pr fixes following compile error on Alpine 3.13:

util.c: In function 'get_seed':
util.c:161:59: error: invalid operands to binary ^ (have 'long int' and 'pthread_t' {aka 'struct __pthread *'})
  161 |     return (time.tv_sec ^ time.tv_nsec ^ (getpid() << 16) ^ pthread_self());
      |             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ^ ~~~~~~~~~~~~~~
      |                                        |                    |
      |                                        long int             pthread_t {aka struct __pthread *}