Add missing override keyword:

* Enable the `suggest-override` warning for gcc
* Fix all functions that were flagged by that warning

src/test/protocol/BuildInfo_test.cpp

@@ -84,7 +84,7 @@ public:
         BEAST_EXPECT(to_packed (from_version (65535, 65535)) == 4294967295);
     }
 
-    void run ()
+    void run () override
     {
         testValues ();
         testStringVersion ();