weston-log: rename the confusing function name weston_compositor_add_log_scope()

There's a function named weston_compositor_add_log_scope()
but it doesn't take a struct weston_compositor argument.

Rename it to weston_log_ctx_add_log_scope(), as
the log_scope is being added to a log_context.

Also, bump libweston_major to 9.

Signed-off-by: Leandro Ribeiro <leandrohr@riseup.net>

libweston/backend-drm/drm.c

@@ -2825,10 +2825,10 @@ drm_backend_create(struct weston_compositor *compositor,
 	b->pageflip_timeout = config->pageflip_timeout;
 	b->use_pixman_shadow = config->use_pixman_shadow;
 
-	b->debug = weston_compositor_add_log_scope(compositor->weston_log_ctx,
-						   "drm-backend",
-						   "Debug messages from DRM/KMS backend\n",
-						    NULL, NULL, NULL);
+	b->debug = weston_log_ctx_add_log_scope(compositor->weston_log_ctx,
+						"drm-backend",
+						"Debug messages from DRM/KMS backend\n",
+						NULL, NULL, NULL);
 
 	compositor->backend = &b->base;