Include wx/math.h in the file using wxRound()

Although it seems to be already included from some other header
implicitly, prefer to also include it explicitly to avoid any breakage
later.

src/osx/window_osx.cpp

@@ -14,6 +14,7 @@
 
 #ifndef WX_PRECOMP
     #include "wx/log.h"
+    #include "wx/math.h"
     #include "wx/app.h"
     #include "wx/utils.h"
     #include "wx/panel.h"