posix: Move gettimeofday declaration to <sys/time.h>

According to POSIX, that's the header which defines this function.
Similarly, nothing in POSIX indicates that <time.h> should have
access to struct timeval, so it's removed (it's made accessible
to <sys/time.h> via <sys/_timeval.h> introduced earlier).

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>