xen: Update Makefile with CONFIG_BLOCK dependency for biomerge.c

Without this dependency we get these compile errors:

linux-next-20101020/drivers/xen/biomerge.c: In function 'xen_biovec_phys_mergeable':
linux-next-20101020/drivers/xen/biomerge.c:8: error: dereferencing pointer to incomplete type
linux-next-20101020/drivers/xen/biomerge.c:9: error: dereferencing pointer to incomplete type
linux-next-20101020/drivers/xen/biomerge.c:11: error: implicit declaration of function '__BIOVEC_PHYS_MERGEABLE'

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Reported-by: Randy Dunlap <randy.dunlap@oracle.com>