Building gnupg2 tests is broken on musl libc

Commit 86baca6e62b33 has introduced the use of the "time_t" type in
tests/gpgscm/scheme.c. The problem is that using this type requires to
#include <time.h>. It happens that with glibc the type is also defined
when including <stdlib.h>, but this is not portable (all man pages
about time_t tell to include time.h).

Regards,
Pierre Labastie(LFS/BLFS editor)