Bug 1188935 - Part 2: enable mDNS module on Android only if API level >= 21; r=mcmanus

netwerk/dns/mdns/libmdns/moz.build

@@ -4,7 +4,7 @@
 # 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/.
 
-if CONFIG['MOZ_WIDGET_TOOLKIT'] == 'android':
+if CONFIG['MOZ_WIDGET_TOOLKIT'] == 'android' and CONFIG['ANDROID_VERSION'] >= '21':
     EXTRA_COMPONENTS += [
         'nsDNSServiceDiscovery.js',
         'nsDNSServiceDiscovery.manifest',