lib/long-options.c: include stdlib.h

[yegorich]

Fixes the following error (compiled with GCC 14.x):

long-options.c:70:11: error: implicit declaration of function ‘exit’ [-Wimplicit-function-declaration]
   70 |           exit (0);
      |           ^~~~

[airtower-luna]

The issue is not limited to glibc, musl needs the include, too. And uClibc at least builds with the #ifdef removed (making the #include unconditional), I haven't done a runtime test with that yet.