volume_id: add support for btrfs

function                                             old     new   delta
volume_id_probe_btrfs                                  -      96     +96
fs2                                                   48      52      +4
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(add/remove: 1/0 grow/shrink: 1/0 up/down: 100/0)             Total: 100 bytes

Signed-off-by: Vladimir Dronnikov <dronnikov@gmail.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/util-linux/volume_id/Kbuild b/util-linux/volume_id/Kbuild
index d78e4ad..3520f24 100644
--- a/util-linux/volume_id/Kbuild
+++ b/util-linux/volume_id/Kbuild
@@ -11,6 +11,7 @@
 lib-$(CONFIG_FEATURE_MOUNT_LABEL)               += get_devname.o
 
 lib-$(CONFIG_VOLUMEID)                          += volume_id.o util.o
+lib-$(CONFIG_FEATURE_VOLUMEID_BTRFS)            += btrfs.o
 lib-$(CONFIG_FEATURE_VOLUMEID_EXT)              += ext.o
 lib-$(CONFIG_FEATURE_VOLUMEID_FAT)              += fat.o
 lib-$(CONFIG_FEATURE_VOLUMEID_HFS)              += hfs.o