patch 9.2.0685: clipboard.c does not get the Wayland CFLAGS on GTK2

patch 9.2.0685: clipboard.c does not get the Wayland CFLAGS on GTK2

Problem:  clipboard.c relies on the GTK include flags from
          pkg-config to pull in the Wayland headers.  This is not
          guaranteed: GTK2 does not add them, and Wayland clipboard
          support is independent of the GUI.
Solution: Add $(WAYLAND_CFLAGS) and $(WAYLAND_CPPFLAGS) to the
          clipboard.c compile rule so it always compiles, consistent
          with the other Wayland files (Christoffer Aasted).

closes: #20576

Signed-off-by: Christoffer Aasted <dezzadk@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>