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);
| ^~~~~~~~~~~~~~~~