Pass the -Wmissing-declarations warning.

This makes it easier to use GTest in projects that build with the
-Wmissing-declarations warning. This fixes the warning in headers and
source files, though not GTest's own tests as it is rather noisy there.

googlemock/test/gmock-matchers_test.cc

@@ -65,10 +65,6 @@
 
 namespace testing {
 
-namespace internal {
-GTEST_API_ string JoinAsTuple(const Strings& fields);
-}  // namespace internal
-
 namespace gmock_matchers_test {
 
 using std::greater;