Addming missing includes in stubs headers.

This is to prepare removing those additional headers in common.h

Change-Id: I707d6d8eaf193eb28195a827c7fec738b968ab3e

src/google/protobuf/stubs/mathutil.h

@@ -34,6 +34,7 @@
 #include <math.h>
 
 #include <google/protobuf/stubs/common.h>
+#include <google/protobuf/stubs/logging.h>
 #include <google/protobuf/stubs/mathlimits.h>
 
 namespace google {

src/google/protobuf/stubs/once.h

@@ -79,6 +79,7 @@
 #define GOOGLE_PROTOBUF_STUBS_ONCE_H__
 
 #include <google/protobuf/stubs/atomicops.h>
+#include <google/protobuf/stubs/callback.h>
 #include <google/protobuf/stubs/common.h>
 
 namespace google {