patch 8.2.5061: C89 requires signal handlers to return void

Problem:    C89 requires signal handlers to return void.
Solution:   Drop RETSIGTYPE and hard-code a void return value.

src/if_cscope.c

@@ -2181,11 +2181,10 @@ cs_read_prompt(int i)
 /*
  * Used to catch and ignore SIGALRM below.
  */
-    static RETSIGTYPE
+    static void
 sig_handler SIGDEFARG(sigarg)
 {
     // do nothing
-    SIGRETURN;
 }
 #endif