systemd: Check for HAVE_POSIX_FALLOCATE

The check for the config variable is using the wrong define
resulting in the #else always being used.

Submitted upstream here:
    http://lists.openembedded.org/pipermail/openembedded-core/2014-April/092000.html

Signed-off-by: Drew Moseley <drew_moseley@mentor.com>