Bug 1416480 - provide a dummy built_in_addons.json file that toolkit is looking for. r=jorgk

mail/extensions/built_in_addons.json

@@ -0,0 +1 @@
+{ "system": [] }

mail/extensions/jar.mn

@@ -0,0 +1,7 @@
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+browser.jar:
+% content browser %content/
+  content/built_in_addons.json    (built_in_addons.json)

mail/extensions/moz.build

@@ -8,3 +8,4 @@ DIRS += [
     'smime',
 ]
 
+JAR_MANIFESTS += ['jar.mn']