gcc: gcc8 only recognizes fallthu

Later gcc is more accepting, but there's at least one gcc8 toolchain that only likes
to hear "fallthru", or it will flag it

lib/core/context.c

@@ -2014,6 +2014,7 @@ next:
 			goto bail;
 		}
 #endif
+		/* fallthru */
 
 	case LWSCD_FINALIZATION:
 

lib/roles/http/client/client-http.c

@@ -143,10 +143,12 @@ lws_http_client_socket_service(struct lws *wsi, struct lws_pollfd *pollfd)
 
 		lws_set_timeout(wsi, NO_PENDING_TIMEOUT, 0);
 
-               /* fallthrough */
+               /* fallthru */
 
 #endif
 
+               /* dummy fallthru to satisfy compiler */
+               /* fallthru */
 	case LRS_H1C_ISSUE_HANDSHAKE:
 
 		lwsl_debug("%s: LRS_H1C_ISSUE_HANDSHAKE\n", __func__);