Avoid format-truncation warning

The existing code gives this error when compiled with GCC 12:

../sway/server.c: In function ‘server_init’:
../sway/server.c:217:75: error: ‘%d’ directive output may be truncated writing between 1 and 11 bytes into a region of size 8 [-Werror=format-truncation=]
  217 |                 snprintf(name_candidate, sizeof(name_candidate), "wayland-%d", i);
      |                                                                           ^~
../sway/server.c:217:66: note: directive argument in the range [-2147483647, 32]
  217 |                 snprintf(name_candidate, sizeof(name_candidate), "wayland-%d", i);
      |                                                                  ^~~~~~~~~~~~
../sway/server.c:217:17: note: ‘snprintf’ output between 10 and 20 bytes into a destination of size 16
  217 |                 snprintf(name_candidate, sizeof(name_candidate), "wayland-%d", i);
      |                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Because i is never negative, this is a false positive, but it is easy to
change i to unsigned to silence the error.

sway/server.c

@@ -213,8 +213,8 @@ bool server_init(struct sway_server *server) {
 
 	// Avoid using "wayland-0" as display socket
 	char name_candidate[16];
-	for (int i = 1; i <= 32; ++i) {
+	for (unsigned int i = 1; i <= 32; ++i) {
-		snprintf(name_candidate, sizeof(name_candidate), "wayland-%d", i);
+		snprintf(name_candidate, sizeof(name_candidate), "wayland-%u", i);
 		if (wl_display_add_socket(server->wl_display, name_candidate) >= 0) {
 			server->socket = strdup(name_candidate);
 			break;