backuptool: Change version to 5.1

Change-Id: Ia48432afeb705edc23b824f65efc753d2b9a7829
diff --git a/prebuilt/bin/backuptool.sh b/prebuilt/bin/backuptool.sh
index a32594b..e72e462 100755
--- a/prebuilt/bin/backuptool.sh
+++ b/prebuilt/bin/backuptool.sh
@@ -5,7 +5,7 @@
 
 export C=/tmp/backupdir
 export S=/system
-export V=5.0
+export V=5.1
 
 # Preserve /system/addon.d in /tmp/addon.d
 preserve_addon_d() {