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>