mpick: use function pointer type for callback

Though POSIX requires this conversion to work correctly (for dlsym),
it is not valid in ISO C, so it is better to just uses the appropriate
function pointer type.

mpick.c

@@ -1466,7 +1466,7 @@ main(int argc, char *argv[])
 
 	xpledge("stdio rpath wpath cpath proc exec", 0);
 
-	void *cb = need_thr ? collect : oneline;
+	void (*cb)(char *) = need_thr ? collect : oneline;
 	if (argc == optind && isatty(0))
 		i = blaze822_loop1(":", cb);
 	else