Android: Ensure RN_FABRIC_ENABLED is defined when building with Fabric (facebook#29939)

Summary:
This pull request tweaks the build script for `jsiruntime` to ensure that `RN_FABRIC_ENABLED` is defined when building for Android with Fabric enabled, and using the JavascriptCore JS engine. Without it, the `createWeakObject` and `lockWeakObject` methods in `JSCRuntime.cpp` will throw an exception.

## Changelog

[Internal] [Changed] - Android: Ensure RN_FABRIC_ENABLED is defined when building with Fabric

Pull Request resolved: facebook#29939

Test Plan: RNTester with JSC now builds and runs on Android with Fabric enabled.

Reviewed By: hramos

Differential Revision: D23671070

Pulled By: fkgozali

fbshipit-source-id: 27e69364a9f709615543be7c20b72b5536443cd8

Author: empyrical

ReactCommon/jsi/Android.mk

@@ -32,4 +32,8 @@ LOCAL_EXPORT_C_INCLUDES := $(LOCAL_PATH)
 LOCAL_CFLAGS := -fexceptions -frtti -O3
 LOCAL_SHARED_LIBRARIES := libfolly_json libjsc glog
 
+ifeq ($(BUILD_FABRIC),true)
+  LOCAL_CFLAGS += -DRN_FABRIC_ENABLED
+endif
+
 include $(BUILD_STATIC_LIBRARY)