Add the explicit include of qpixmapcache.h to qlabel.cpp

The include is required since code now accesses the respective types.

Amends 7a238e1225f49b81772516ed5d0a5a4f4f2e9268

Fixes: QTBUG-136079
Pick-to: 6.9
Change-Id: I3c8fb8cd5b46875613b866bcb9c6bf7cded926a3
Reviewed-by: Christian Ehrlicher <ch.ehrlicher@gmx.de>

src/widgets/widgets/qlabel.cpp

@@ -3,6 +3,7 @@
 
 #include "qpainter.h"
 #include "qevent.h"
+#include "qpixmapcache.h"
 #include "qstyle.h"
 #include "qstyleoption.h"
 #include "qlabel_p.h"