GUI: Fix endless loop in previous commit.

The iterator was not advanced in the loop. So the code worked when
I ran it in Linux as the first item was matching.

Thanks for Robert for spotting and reporting it.

gui/platforms.cpp

@@ -56,6 +56,7 @@ Platform& Platforms::get(Settings::PlatformType platform)
         {
             return *iter;
         }
+        ++iter;
     }
     return mPlatforms.first();
 }