Fix MSVC build.

common/eda_draw_frame.cpp

@@ -52,6 +52,7 @@
 #include <tool/actions.h>
 #include <tool/action_toolbar.h>
 #include <tool/common_tools.h>
+#include <tool/grid_helper.h>
 #include <tool/grid_menu.h>
 #include <tool/selection_conditions.h>
 #include <tool/tool_dispatcher.h>
@@ -525,6 +526,12 @@ void EDA_DRAW_FRAME::SetGridOverrides( bool aOverride )
 }
 
 
+std::unique_ptr<GRID_HELPER> EDA_DRAW_FRAME::MakeGridHelper()
+{
+    return nullptr;
+}
+
+
 void EDA_DRAW_FRAME::UpdateZoomSelectBox()
 {
     if( m_zoomSelectBox == nullptr )

include/eda_draw_frame.h

@@ -151,7 +151,7 @@ public:
     virtual const VECTOR2I& GetGridOrigin() const = 0;
     virtual void            SetGridOrigin( const VECTOR2I& aPosition ) = 0;
 
-    virtual std::unique_ptr<GRID_HELPER> MakeGridHelper() { return nullptr; }
+    virtual std::unique_ptr<GRID_HELPER> MakeGridHelper();
 
     /**
      * Return the nearest \a aGridSize location to \a aPosition.