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: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Matthew Wilcox <matthew.r.wilcox@intel.com>
diff --git a/drivers/xen/biomerge.c b/drivers/xen/biomerge.c
index ba6eda4..18c1bb6 100644
--- a/drivers/xen/biomerge.c
+++ b/drivers/xen/biomerge.c
@@ -11,3 +11,4 @@
 	return __BIOVEC_PHYS_MERGEABLE(vec1, vec2) &&
 		((mfn1 == mfn2) || ((mfn1+1) == mfn2));
 }
+EXPORT_SYMBOL(xen_biovec_phys_mergeable);