Add missing header file when HAVE_FORK
Add missing header file when HAVE_FORK

Function _exit(int) is included in unistd.h. Also fork() is here.
Functions exit(int) (C89) and _Exit(int) (C99) are in stdlib.h.

Signed-off-by: Mikko Johannes Koivunalho <b1070ee6d40473e0bf54e1be38982cfcedc3436a@iki.fi>

libcheck/check,libcheck/check,libcheck/check,libcheck/check,libcheck/check