Btrfs: Makefile changes to optionally include btrfs integrity check

If the btrfs integrity check is enabled, the files required to
implement the checks are included in the build.
Signed-off-by: Stefan Behrens <sbehrens@giantdisaster.de>

fs/btrfs/Makefile

@@ -11,3 +11,4 @@ btrfs-y += super.o ctree.o extent-tree.o print-tree.o root-tree.o dir-item.o \
 	   reada.o backref.o
 
 btrfs-$(CONFIG_BTRFS_FS_POSIX_ACL) += acl.o
+btrfs-$(CONFIG_BTRFS_FS_CHECK_INTEGRITY) += check-integrity.o