posix: Don't require struct itimerspec in posix/time.h

struct itimerspec is only required to be available when building with
_POSIX_C_SOURCE defined. Applications including posix/time.h without
defining _POSIX_C_SOURCE will need to have a forward declaration of struct
itimerspec even if they don't use the functions requiring that type.

Signed-off-by: Keith Packard <keithp@keithp.com>