qpushbutton_p.h: add missing include guard

Change-Id: I255550e7235dcc8d2170f97e08752c2cb7cea624
Reviewed-by: Thorbjørn Lund Martsum <tmartsum@gmail.com>
Reviewed-by: Sergio Ahumada <sergio.ahumada@digia.com>

src/widgets/widgets/qpushbutton_p.h

@@ -39,6 +39,9 @@
 **
 ****************************************************************************/
 
+#ifndef QPUSHBUTTON_P_H
+#define QPUSHBUTTON_P_H
+
 #include "private/qabstractbutton_p.h"
 
 //
@@ -88,3 +91,5 @@ public:
 };
 
 QT_END_NAMESPACE
+
+#endif // QPUSHBUTTON_P_H