metag: export metag_code_cache_flush_all

Various file systems indirectly use metag_code_cache_flush_all(), so
when they're built as modules we get build errors like the following:

ERROR: "metag_code_cache_flush_all" [fs/xfs/xfs.ko] undefined!

Therefore export this function to modules to fix the errors. This was
hit by a randconfig build.

Signed-off-by: James Hogan <james.hogan@imgtec.com>