include/signal.h: Add commented out definition of the si_addr field.

include/signal.h

@@ -329,6 +329,9 @@ struct siginfo
   pid_t        si_pid;       /* Sending task ID */
   int          si_status;    /* Exit value or signal (SIGCHLD only). */
 #endif
+#if 0                        /* Not implemented */
+  FAR void    *si_addr;      /* Report address with SIGFPE, SIGSEGV, or SIGBUS */
+#endif
 };
 
 #ifndef __SIGINFO_T_DEFINED