Use the named SINCE_VERSION defines for version checks

To make our code more clear.
diff --git a/src/input.c b/src/input.c
index aaa2223..b6cd7df 100644
--- a/src/input.c
+++ b/src/input.c
@@ -1829,7 +1829,7 @@
 		caps |= WL_SEAT_CAPABILITY_TOUCH;
 
 	wl_seat_send_capabilities(resource, caps);
-	if (version >= 2)
+	if (version >= WL_SEAT_NAME_SINCE_VERSION)
 		wl_seat_send_name(resource, seat->seat_name);
 }