Fix build on Windows

The competition between Qt and SDL to redefine main() on Windows (for
portability purposes and the WinMain entry point for GUI applications)
ends up badly when Qt is included before SDL. Thus make sure to include
SDL before any other header.

See #1169.

client/gui_main.cpp

@@ -8,7 +8,7 @@
  see https://www.gnu.org/licenses/.
 **************************************************************************/
 
-#include "mapview_common.h"
+// Keep this first. In particular before any Qt include.
 #ifdef AUDIO_SDL
 #include <SDL2/SDL.h>
 #endif // AUDIO_SDL
@@ -21,6 +21,7 @@
 // client
 #include "client_main.h"
 #include "clinet.h"
+#include "mapview_common.h"
 #include "mapview_g.h"
 #include "options.h"
 #include "sprite.h"