Replace direct use of usleep()

This avoids a deprecation warning when using usleep() with MinGW32.

main.c: In function 'check_pause':
main.c:158:4: warning: 'usleep' is deprecated [-Wdeprecated-declarations]
  158 |    usleep(100000);
      |    ^~~~~~
In file included from main.c:19:
d:\mingw\include\unistd.h:104:5: note: declared here
  104 | int usleep( useconds_t period ){ return __mingw_sleep( 0, 1000 * period ); }
      |     ^~~~~~

libxmp / xmp-cli

Replace direct use of usleep() #38