libweston: add missing struct weston_renderer forward declaration

struct weston_renderer is defined in libweston-internal.h, which is not
included from libweston.h. Add the missing forward declaration for the
renderer pointer stored in struct weston_compositor.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>

include/libweston/libweston.h

@@ -1197,6 +1197,7 @@ struct weston_debug_compositor;
 struct weston_color_manager;
 struct weston_dmabuf_feedback;
 struct weston_dmabuf_feedback_format_table;
+struct weston_renderer;
 
 /** Main object, container-like structure which aggregates all other objects.
  *