Enable rtmp in external demo app with extensions

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=168007345

demo/build.gradle

@@ -62,4 +62,5 @@ dependencies {
     withExtensionsCompile project(path: modulePrefix + 'extension-ima')
     withExtensionsCompile project(path: modulePrefix + 'extension-opus')
     withExtensionsCompile project(path: modulePrefix + 'extension-vp9')
+    withExtensionsCompile project(path: modulePrefix + 'extension-rtmp')
 }