xen: using EXPORT_SYMBOL requires including export.h

Fix these warnings:

  drivers/xen/biomerge.c:14:1: warning: data definition has no type or storage class [enabled by default]
  drivers/xen/biomerge.c:14:1: warning: type defaults to 'int' in declaration of 'EXPORT_SYMBOL' [-Wimplicit-int]
  drivers/xen/biomerge.c:14:1: warning: parameter names (without types) in function declaration [enabled by default]

And this build error:

  ERROR: "xen_biovec_phys_mergeable" [drivers/block/nvme.ko] undefined!

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>