posix: sysconf: correct typo on _SC_MAPPED_FILE(S)

The standard macro is plural - so _SC_MAPPED_FILES - which
corresponds to _POSIX_MAPPED_FILES.

In Zephyr, we previously had _SC_MAPPED_FILE and
_POSIX_MAPPED_FILE, so it's a simple correction.

Signed-off-by: Chris Friedt <cfriedt@tenstorrent.com>