Add missing "#pragma once".

As a general rule, all C++ header files should start with #pragma once. Add a few that were missing.

Don't touch third party code such as "cutest.h" unless really needed. Also leave "example.h" alone. It is actually source code but has .h suffix to make Doxygen include the file verbatim.

native/src/test/mock.h

@@ -12,6 +12,8 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+#pragma once
+
 #include "../api.h"
 
 #ifdef _MSC_VER