security-context: constify lookup_client()

This is worth doing despite wl_client_get_destroy_listener() incorrectly
accepting a mutable wl_client pointer since this lookup_client()
function will primarily be used with a wl_display_global_filter_func_t
which only provides a const wl_client pointer.

Work around this libwayland API wart in wlroots so compositors don't
have to.

include/wlr/types/wlr_security_context_v1.h

@@ -50,6 +50,6 @@ struct wlr_security_context_v1_commit_event {
 struct wlr_security_context_manager_v1 *wlr_security_context_manager_v1_create(
 	struct wl_display *display);
 const struct wlr_security_context_v1_state *wlr_security_context_manager_v1_lookup_client(
-	struct wlr_security_context_manager_v1 *manager, struct wl_client *client);
+	struct wlr_security_context_manager_v1 *manager, const struct wl_client *client);
 
 #endif

types/wlr_security_context_v1.c

@@ -429,8 +429,8 @@ struct wlr_security_context_manager_v1 *wlr_security_context_manager_v1_create(
 }
 
 const struct wlr_security_context_v1_state *wlr_security_context_manager_v1_lookup_client(
-		struct wlr_security_context_manager_v1 *manager, struct wl_client *client) {
+		struct wlr_security_context_manager_v1 *manager, const struct wl_client *client) {
-	struct wl_listener *listener = wl_client_get_destroy_listener(client,
+	struct wl_listener *listener = wl_client_get_destroy_listener((struct wl_client *)client,
 		security_context_client_handle_destroy);
 	if (listener == NULL) {
 		return NULL;