Fix prefab prefix for fabricjni

Summary:
The `fabricjni` prefab module was missing the `react/fabric` prefix.
I'm adding it here.
Ref: reactwg/react-native-releases#41 (reply in thread)

Changelog:
[Internal] [Changed] - Fix prefab prefix for fabricjni

Differential Revision: D42047434

fbshipit-source-id: e4e34992fb3e0f8c562db0e331961b34ad9aed31

ReactAndroid/build.gradle

@@ -122,7 +122,7 @@ final def preparePrefab = tasks.register("preparePrefab", PreparePrefabHeadersTa
             new PrefabPreprocessingEntry(
                 "fabricjni",
                 [
-                    new Pair("src/main/jni/react/fabric", ""),
+                    new Pair("src/main/jni/react/fabric", "react/fabric/"),
                     new Pair("src/main/jni/react/cxxcomponents", "react/cxxcomponents/")
                 ]
             ),