posix: consolidate included C sources.

* ext/include/strlcpy.c: Add multiple include guards.
* ext/posix/_helpers.c: Move from here...
* ext/include/_helpers.c: ...to here.
* local.mk (EXTRA_ext_posix_posix_la_SOURCES)
(EXTRA_DIST): Adjust accordingly.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>