Utils: Turn StyleHelper into a namespace

It was in fact just a bag of static functions. Most importantly, it
fixes a previouls introduced build error.

Amends 5975657e77

Change-Id: Ia7de8bdcf91e1763f9d3b435316a5df9993717de
Reviewed-by: Cristian Adam <cristian.adam@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>

src/plugins/coreplugin/manhattanstyle.cpp

@@ -154,7 +154,7 @@ QPixmap getDeletePixmap(bool enabled,
 {
     using Utils::Theme;
     using Utils::creatorTheme;
-    using Utils::StyleHelper;
+    using namespace Utils::StyleHelper;
 
     const double xRatio = 19;
     const double yRatio = 9;