Declare struct wlr_surface in a less weird place

wlr_compositor.h contains references to `struct wlr_surface` in function
arguments before it actually defines it.  This generally works because
wlr_compositor.h includes wlr_output.h which contains a
forward-declaration for `struct wlr_surface` (despite not actually
referencing it).

This is all pretty weird, and gives very confusing errors if you manage
to end up with wlr_output.h including wlr_compositor.h (eg. via an
indirect route) so make it less weird.

include/wlr/types/wlr_compositor.h

@@ -18,6 +18,8 @@
 #include <wlr/util/addon.h>
 #include <wlr/util/box.h>
 
+struct wlr_surface;
+
 enum wlr_surface_state_field {
 	WLR_SURFACE_STATE_BUFFER = 1 << 0,
 	WLR_SURFACE_STATE_SURFACE_DAMAGE = 1 << 1,

include/wlr/types/wlr_cursor.h

@@ -14,6 +14,7 @@
 #include <wlr/types/wlr_output.h>
 
 struct wlr_input_device;
+struct wlr_surface;
 struct wlr_xcursor_manager;
 
 /**

include/wlr/types/wlr_output.h

@@ -296,8 +296,6 @@ struct wlr_output_event_request_state {
 	const struct wlr_output_state *state;
 };
 
-struct wlr_surface;
-
 void wlr_output_create_global(struct wlr_output *output, struct wl_display *display);
 void wlr_output_destroy_global(struct wlr_output *output);
 /**