Make wxGCDC::GetGraphicsContext() const.

There is no reason for this accessor to not be const and it prevents other
methods using it from being const too, so change this.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@68179 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/common/dcgraph.cpp

@@ -123,7 +123,7 @@ wxGCDC::~wxGCDC()
 {
 }
 
-wxGraphicsContext* wxGCDC::GetGraphicsContext()
+wxGraphicsContext* wxGCDC::GetGraphicsContext() const
 {
     if (!m_pimpl) return NULL;
     wxGCDCImpl *gc_impl = (wxGCDCImpl*) m_pimpl;