Doc: Fix CustomStyle snippet

Add missing semicolon, include QtWidgets instead of QtGui and
use cast for a const pointer because the example with casting
relates to the code above where 'widget' is const.

Task-number: QTBUG-31736
Change-Id: I5cb6d33ec692deaf41e9a1be9e36e752a8da8739
Reviewed-by: Venugopal Shivashankar <venugopal.shivashankar@digia.com>

src/widgets/doc/snippets/customstyle/customstyle.cpp

@@ -45,7 +45,7 @@
 CustomStyle::CustomStyle()
 {
 //! [0]
-    QSpinBox *spinBox = qobject_cast<QSpinBox *>(widget);
+    const QSpinBox *spinBox = qobject_cast<const QSpinBox *>(widget);
     if (spinBox) {
 //! [0] //! [1]
     }

src/widgets/doc/snippets/customstyle/customstyle.h

@@ -49,7 +49,7 @@ class CustomStyle : public QProxyStyle
     Q_OBJECT
 
 public:
-    CustomStyle()
+    CustomStyle();
     ~CustomStyle() {}
 
     void drawPrimitive(PrimitiveElement element, const QStyleOption *option,

src/widgets/doc/snippets/customstyle/customstyle.pro

@@ -1,2 +1,3 @@
+QT += widgets
 HEADERS += customstyle.h
 SOURCES += customstyle.cpp main.cpp

src/widgets/doc/snippets/customstyle/main.cpp

@@ -39,7 +39,7 @@
 ****************************************************************************/
 
 //! [using a custom style]
-#include <QtGui>
+#include <QtWidgets>
 
 #include "customstyle.h"
 

src/widgets/doc/snippets/customviewstyle.cpp

@@ -38,7 +38,7 @@
 **
 ****************************************************************************/
 
-#include <QtGui>
+#include <QtWidgets>
 
 #include "customviewstyle.h"