Fixed compile on Windows without precompiled headers

Added missing header.

Compile failure is only reproducible when precompiled headers are
disabled.

Change-Id: I0fb2e51b3352ebad3b7b5b58898347605b8d5090
Reviewed-by: Toby Tomkins <toby.tomkins@nokia.com>
Reviewed-by: Kalle Lehtonen <kalle.ju.lehtonen@nokia.com>

src/gui/image/qpixmap_win.cpp

@@ -39,6 +39,7 @@
 **
 ****************************************************************************/
 
+#include "qbitmap.h"
 #include "qpixmap.h"
 #include <qpa/qplatformpixmap.h>
 #include "qpixmap_raster_p.h"