Fix link issue on MSVC (thanks @mroszko)

wxScrolledCanvas is not properly exported, so when we
add a wxGrid it has a chance of creating linker errors.
Including the wx/grid.h ensures that the linker does not
see two conflicting symbols between gal and the other
place where wxGrid is used.

include/class_draw_panel_gal.h

@@ -30,6 +30,7 @@
 
 #include <wx/window.h>
 #include <wx/timer.h>
+#include <wx/grid.h> // needed for MSVC to see wxScrolledCanvas indirectly exported
 #include <math/box2.h>
 #include <math/vector2d.h>
 #include <msgpanel.h>