Fix MSVC C4840 warning

va_start requires trivially copy able classes.

wxString is not trivial. The correct thing to do is to use wxChar*

pcbnew/drc/drc_test_provider.cpp

@@ -102,7 +102,7 @@ bool DRC_TEST_PROVIDER::reportPhase( const wxString& aMessage )
 }
 
 
-void DRC_TEST_PROVIDER::reportAux( wxString fmt, ... )
+void DRC_TEST_PROVIDER::reportAux( const wxChar* fmt, ... )
 {
     va_list vargs;
     va_start( vargs, fmt );

pcbnew/drc/drc_test_provider.h

@@ -103,7 +103,7 @@ protected:
     int forEachGeometryItem( const std::vector<KICAD_T>& aTypes, LSET aLayers,
                              const std::function<bool(BOARD_ITEM*)>& aFunc );
 
-    virtual void reportAux( wxString fmt, ... );
+    virtual void reportAux( const wxChar* fmt, ... );
     virtual void reportViolation( std::shared_ptr<DRC_ITEM>& item, const VECTOR2I& aMarkerPos,
                                   int aMarkerLayer );
     virtual bool reportProgress( int aCount, int aSize, int aDelta );