wlr_gamma_control: add missing forward declarations

Fixes an error seen in labwc CI builds with -Werror:

    ../subprojects/wlroots/include/wlr/types/wlr_gamma_control_v1.h:44:16:
    error: ‘struct wlr_output_state’ declared inside parameter list
    will not be visible outside of this definition or declaration [-Werror]
       44 |         struct wlr_output_state *output_state);
          |                ^~~~~~~~~~~~~~~~

include/wlr/types/wlr_gamma_control_v1.h

@@ -3,6 +3,9 @@
 
 #include <wayland-server-core.h>
 
+struct wlr_output;
+struct wlr_output_state;
+
 struct wlr_gamma_control_manager_v1 {
 	struct wl_global *global;
 	struct wl_list controls; // wlr_gamma_control_v1.link