Fix testcase compile failure under gcc 4.6:

google/protobuf/map_test.cc:88:16: error: expected ‘;’ at end of member declaration
google/protobuf/map_test.cc:88:18: error: ‘override’ does not name a type
.

src/google/protobuf/map_test.cc

@@ -85,7 +85,7 @@ class MapImplTest : public ::testing::Test {
     EXPECT_TRUE(map_.empty());
     EXPECT_EQ(0, map_.size());
   }
-  ~MapImplTest() override {}
+  ~MapImplTest() {}
 
   void ExpectSingleElement(int32 key, int32 value) {
     EXPECT_FALSE(map_.empty());