[XFS] Remove STATIC which was missing from prior manual merge

Removes STATIC on xfs_freeze function which was not manually
applied for SGI-Modid: xfs-linux-melb:xfs-kern:29504a.

Reviewed-by: Lachlan McIlroy <lachlan@sgi.com>
Signed-off-by: Tim Shimmin <tes@sgi.com>