maint: convert remove a K&R declaration

This function is hidden unless EVAL_TRACE is defined for debugging.

* src/expr.c (trace): Convert to a prototype instead of K&R definition
to avoid -Werror=strict-prototypes. Make the argument const to avoid
-Werror=discarded-qualifiers.

src/expr.c

@@ -552,8 +552,7 @@ require_more_args (void)
 /* Print evaluation trace and args remaining.  */
 
 static void
-trace (fxn)
-     char *fxn;
+trace (char const *fxn)
 {
   char **a;