Add +DEBUG to the raw version string for DEBUG builds.

This will show up in the rpc server_info command.
There is no impact on the version string for release builds.

src/ripple/data/protocol/BuildInfo.cpp

@@ -37,6 +37,9 @@ char const* BuildInfo::getRawVersionString ()
     //
     //  http://semver.org/
     //
+#ifdef DEBUG
+        "+DEBUG"
+#endif
     //--------------------------------------------------------------------------
     ;