btrfs: tiny makefile cleanup

use normal kbuild syntax to build acl.o conditinally and remove comment
out lines.
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Signed-off-by: NChris Mason <chris.mason@oracle.com>

fs/btrfs/Makefile

@@ -8,13 +8,7 @@ btrfs-y := super.o ctree.o extent-tree.o print-tree.o root-tree.o dir-item.o \
 	   extent_map.o sysfs.o struct-funcs.o xattr.o ordered-data.o \
 	   extent_io.o volumes.o
 
-ifeq ($(CONFIG_FS_POSIX_ACL),y)
-btrfs-y += acl.o
-endif
-#btrfs-y := ctree.o disk-io.o radix-tree.o extent-tree.o print-tree.o \
-#	  root-tree.o dir-item.o hash.o file-item.o inode-item.o \
-#	  inode-map.o \
-
+btrfs-$(CONFIG_FS_POSIX_ACL)	+= acl.o
 else
 
 # Normal Makefile