Xen: Export xen_biovec_phys_mergeable

When Xen is enabled, using BIOVEC_PHYS_MERGEABLE in a module
causes xen_biovec_phys_mergeable to be referenced, so it needs
to be exported.
Acked-by: NJens Axboe <axboe@kernel.dk>
Signed-off-by: NMatthew Wilcox <matthew.r.wilcox@intel.com>

drivers/xen/biomerge.c

@@ -11,3 +11,4 @@ bool xen_biovec_phys_mergeable(const struct bio_vec *vec1,
 	return __BIOVEC_PHYS_MERGEABLE(vec1, vec2) &&
 		((mfn1 == mfn2) || ((mfn1+1) == mfn2));
 }
+EXPORT_SYMBOL(xen_biovec_phys_mergeable);