xen: provide a prototype for xen_biovec_phys_mergeable in xen.h

Having multiple externs in arch headers is not a good way to provide
a common interface.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

include/xen/xen.h

@@ -39,4 +39,8 @@ extern uint32_t xen_start_flags;
 #define xen_initial_domain()	(0)
 #endif	/* CONFIG_XEN_DOM0 */
 
+struct bio_vec;
+bool xen_biovec_phys_mergeable(const struct bio_vec *vec1,
+		const struct bio_vec *vec2);
+
 #endif	/* _XEN_XEN_H */