Advertise wl_seat version 7

This does not require any code changes, as we already copy the keymap
data separately for each client.

For details, see 905c0a341d

types/seat/wlr_seat.c

@@ -12,7 +12,7 @@
 #include "types/wlr_seat.h"
 #include "util/signal.h"
 
-#define SEAT_VERSION 6
+#define SEAT_VERSION 7
 
 static void seat_handle_get_pointer(struct wl_client *client,
 		struct wl_resource *seat_resource, uint32_t id) {