[third_party][googletest] Make gtest-spi.h public.

* gtest-spi.h is needed for the EXPECT_(NON)FATAL_FAILURE macros; see
<https://github.com/google/googletest/blob/master/googletest/docs/AdvancedGuide.md#catching-failures>
for more information.

* Needed by
<https://fuchsia-review.googlesource.com/c/peridot/+/146159/17/public/lib/async/cpp/future_unittest.cc>.

Change-Id: Ie4803ad8be336fb812159dc0e543120e39d86604
diff --git a/BUILD.gn b/BUILD.gn
index 4acfa53..db255e7 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -15,13 +15,13 @@
   testonly = true
   public = [
     "googletest/include/gtest/gtest.h",
+    "googletest/include/gtest/gtest-spi.h",
   ]
   sources = [
     "googletest/include/gtest/gtest-death-test.h",
     "googletest/include/gtest/gtest-message.h",
     "googletest/include/gtest/gtest-param-test.h",
     "googletest/include/gtest/gtest-printers.h",
-    "googletest/include/gtest/gtest-spi.h",
     "googletest/include/gtest/gtest-test-part.h",
     "googletest/include/gtest/gtest-typed-test.h",
     "googletest/include/gtest/gtest_pred_impl.h",