flambda-backend: Do not export "fail.h" from "signals.h" in runtime4 (#2979)

remove [#include "fail.h"] in [runtime4/signals.h]

Signed-off-by: David Vulakh <dvulakh@janestreet.com>

Author: dvulakh

runtime4/caml/signals.h

@@ -25,7 +25,6 @@
 #endif
 #include "misc.h"
 #include "mlvalues.h"
-#include "fail.h"
 
 #ifdef __cplusplus
 extern "C" {