use namespace GFLAGS_NAMESPACE instead namespace gflags

only signalhandler_unittest.cc uses "using namespace gflags".
others use "using namespace GFLAGS_NAMESPACE", so
signalhandler_unittest.cc does the same way.

fixes #62

src/signalhandler_unittest.cc

@@ -43,7 +43,7 @@
 
 #ifdef HAVE_LIB_GFLAGS
 #include <gflags/gflags.h>
-using namespace gflags;
+using namespace GFLAGS_NAMESPACE;
 #endif
 
 using namespace GOOGLE_NAMESPACE;