file: Turns out we need xlocal.h protection

It turns out that we still need xlocal.h protection for when we're
cross building on Linux. Linux doesn't have this file, but os/x
does. Before, we'd assume we didn't have it, like old FreeBSD, when
cross compiling. After the latest update, all that code was removed so
cross compiling needs to be handled separaetly. Do so by defining
HAVE_XLOCALE_H only when we're not building on linux. This allows us
to build the mkmagic build tool needed to build file(1).

Sponsored by: Netflix
Reviewed by: jrtc27
Differential Revision: https://reviews.freebsd.org/D33741