wayland: Add MetaWaylandTabletTool

This struct holds the server-side information of a wl_tablet_tool, which
represents an specific tool of an specific tablet, and is unique as such.

src/wayland/meta-wayland-types.h

@@ -38,6 +38,7 @@ typedef struct _MetaWaylandDataOffer MetaWaylandDataOffer;
 typedef struct _MetaWaylandDataDevice MetaWaylandDataDevice;
 
 typedef struct _MetaWaylandTabletSeat MetaWaylandTabletSeat;
+typedef struct _MetaWaylandTabletTool MetaWaylandTabletTool;
 typedef struct _MetaWaylandTablet MetaWaylandTablet;
 
 typedef struct _MetaWaylandBuffer MetaWaylandBuffer;