Consistently declare wxCursor(wxImage) ctor when wxUSE_IMAGE==1

wxGTK already did it like this, but also only declared ctor not taking
wxImage in this case, while wxMSW and wxMac declared this ctor in any
case, even when wxUSE_IMAGE==0, but didn't define it then.

This doesn't matter much anyhow, as the build with wxUSE_IMAGE==0 is
clearly broken and these changes are not enough to fix it, but be at
least somewhat consistent and:

1. Don't declare methods using a class which is not available at all.
2. Do define methods using wxImage only internally, even if they do
   nothing when it's not available.

No real changes.

include/wx/gtk/cursor.h

@@ -25,10 +25,10 @@ public:
 #endif
 #if wxUSE_IMAGE
     wxCursor( const wxImage & image );
+#endif // wxUSE_IMAGE
     wxCursor(const wxString& name,
              wxBitmapType type = wxCURSOR_DEFAULT_TYPE,
              int hotSpotX = 0, int hotSpotY = 0);
-#endif
     wxCursor( const char bits[], int width, int height,
               int hotSpotX = -1, int hotSpotY = -1,
               const char maskBits[] = NULL,