include/posix: incorporate toolchain-provided time.h header

Some of what's supposed to be in <time.h> was lost because this header
attempts to define everything using more primitive include files.
Instead incorporate the contents from the toolchain-provided header.
Any gaps should be picked up by the legacy content present in this
file, which should not conflict with the toolchain header.

Signed-off-by: Peter Bigot <peter.bigot@nordicsemi.no>