Convert fb_xplat_cxx to Skylark

Reviewed By: ttsugriy

Differential Revision: D6937683

fbshipit-source-id: 472c95b393d054c29413a46b86bf03559aff8741

Author: Jakub Grzmiel

Diff for: ReactCommon/cxxreact/tests/BUCK

@@ -12,7 +12,7 @@ TEST_SRCS = [
 
 if not IS_OSS_BUILD:
     load("@xplat//configurations/buck/android:jni_instrumentation_test", "jni_instrumentation_test_lib")
-    load("@xplat//configurations/buck:fb_xplat_cxx", "cxx_test")
+    load("@xplat//configurations/buck:fb_xplat_cxx.bzl", "cxx_test")
     load("@xplat//configurations/buck:default_platform_defs.bzl", "APPLE")
     jni_instrumentation_test_lib(
         name = 'tests',