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>

drivers/xen/Makefile

-obj-y	+= grant-table.o features.o events.o manage.o biomerge.o
+obj-y	+= grant-table.o features.o events.o manage.o
 obj-y	+= xenbus/
 
 nostackp := $(call cc-option, -fno-stack-protector)
 CFLAGS_features.o			:= $(nostackp)
 
+obj-$(CONFIG_BLOCK)		+= biomerge.o
 obj-$(CONFIG_HOTPLUG_CPU)	+= cpu_hotplug.o
 obj-$(CONFIG_XEN_XENCOMM)	+= xencomm.o
 obj-$(CONFIG_XEN_BALLOON)	+= balloon.o