Warn when trying to load an icon without a matching icon engine

A typical case is trying to load an SVG icon without the QtSvg module
built. You want to know what's going on instead of trying to debug why
the icon doesn't produce any valid images.

Change-Id: I4418ad758a1232f1394058368c50e0d87235271e
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

src/gui/image/qicon.cpp

@@ -1049,6 +1049,8 @@ static QIconEngine *iconEngineFromSuffix(const QString &fileName, const QString
             }
         }
     }
+    qWarning("Could not find icon engine for suffix '%s' of file '%s'",
+        qUtf8Printable(suffix), qUtf8Printable(fileName));
     return nullptr;
 }