Fix harmless unused variable warning.

The value of wxColour::Alpha() was assigned to a temporary variable which
wasn't used afterwards and Mac OS g++ warned about it.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@61984 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

tests/graphics/colour.cpp

@@ -55,7 +55,7 @@ std::ostream& operator<<(std::ostream& os, const wxColour& c)
 
     if ( const unsigned char a = c.Alpha() )
     {
-        os << ", " << ColourChannel(c.Alpha());
+        os << ", " << ColourChannel(a);
     }
 
     os << ")";