Solves a warning or even a compilation break on rocky 8 (tx Shailendra Shukla)

diff -ruN greylisting-0.5-original/greylisting.c greylisting-0.5/greylisting.c
--- greylisting-0.5-original/greylisting.c	2015-05-19 21:07:27.000000000 +0200
+++ greylisting-0.5/greylisting.c	2025-03-16 13:08:31.822506866 +0100
@@ -19,7 +19,7 @@
  * Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
  */
 
-#define _BSD_SOURCE
+#define _DEFAULT_SOURCE
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>