libc/common: Define _POSIX_C_SOURCE to gain access to POSIX functions

ctime.c and localtime_r_utc.c may use POSIX-only functions; to ensure those
are visible from a POSIX-conforming C library, define _POSIX_C_SOURCE in
these source files.

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