Do variable length arrays the C99 way

Variable length arrays are supported by C99, but having it denoted as "1" confused the compiler and is not defined.

Note that because we don't get the inferred NULL terminator, we have to increase the malloc size by one.

include/sudo_event.h

@@ -64,7 +64,7 @@ typedef void (*sudo_ev_callback_t)(int fd, int what, void *closure);
 struct sudo_ev_siginfo_container {
     void *closure;
     siginfo_t *siginfo;
-    char si_buf[1];
+    char si_buf[];
 };
 
 /* Member of struct sudo_event_base. */