releasetools: Add signapk as required modules

* common.py requires the signapk jar

Test: m otapackage ( before it was broken )
Change-Id: Ic0cff0a21258cb1e7d1ced5553a940dcf5cd6d04

tools/releasetools/Android.bp

@@ -188,6 +188,7 @@ python_library_host {
         "imgdiff",
         "minigzip",
         "mkbootfs",
+        "signapk",
     ],
 }