releasetools: Add signapk as required modules

* common.py requires the signapk jar

Test: m otapackage ( before it was broken )
Change-Id: Ic0cff0a21258cb1e7d1ced5553a940dcf5cd6d04
diff --git a/tools/releasetools/Android.bp b/tools/releasetools/Android.bp
index 90a6485..f29ec90 100644
--- a/tools/releasetools/Android.bp
+++ b/tools/releasetools/Android.bp
@@ -188,6 +188,7 @@
         "imgdiff",
         "minigzip",
         "mkbootfs",
+        "signapk",
     ],
 }