GTK syle: initialize all members of GdkColor

Coverity rightfully complains that we're copying a struct with an
uninitialized member, triggering undefined behavior.

Change-Id: I7635b859eb11e5eb9b825df8e23b453116059892
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>

src/widgets/styles/qgtkstyle.cpp

@@ -297,6 +297,7 @@ static QColor mergedColors(const QColor &colorA, const QColor &colorB, int facto
 static GdkColor fromQColor(const QColor &color)
 {
     GdkColor retval;
+    retval.pixel = 0;
     retval.red = color.red() * 255;
     retval.green = color.green() * 255;
     retval.blue = color.blue() * 255;