Fix MinGW compiler warning in qwindowsxpstyle.cpp.

qwindowsxpstyle.cpp: In static member function 'static QRect QWindowsXPStylePrivate::scrollBarGripperBounds(QStyle::State, const QWidget*, XPThemeData*)':styles\qwindowsxpstyle.cpp:2542:45: warning: suggest parentheses around '&&' within '||' [-Wparentheses]

Change-Id: I6a4b62057a612fa9234744e892950c959c513d15
Reviewed-by: Kai Koehne <kai.koehne@theqtcompany.com>