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.
diff --git a/src/if_cscope.c b/src/if_cscope.c
index f373952..5d24533 100644
--- a/src/if_cscope.c
+++ b/src/if_cscope.c
@@ -2181,11 +2181,10 @@
 /*
  * Used to catch and ignore SIGALRM below.
  */
-    static RETSIGTYPE
+    static void
 sig_handler SIGDEFARG(sigarg)
 {
     // do nothing
-    SIGRETURN;
 }
 #endif