libc/picolibc: Remove the global definition of _POSIX_C_SOURCE

This was necessary to get Picolibc to expose the whole Zephyr C library
API, but current versions of the SDK use a version of Picolibc with
built-in Zephyr support.

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