Add missing int return type for the print_banner function

This avoids relying on an implicit int type, a language feature
removed in C99.

diff --git a/nbtscan.c b/nbtscan.c
index 107a8635ec026997..82044c1e04d76411 100644
--- a/nbtscan.c
+++ b/nbtscan.c
@@ -19,7 +19,7 @@
 
 int quiet=0;
 
-print_banner() {
+int print_banner() {
   printf("\nNBTscan version 1.5.1. Copyright (C) 1999-2003 Alla Bezroutchko.\n");
   printf("This is a free software and it comes with absolutely no warranty.\n");
   printf("You can use, distribute and modify it under terms of GNU GPL.\n\n");