mmap: Define _DEFAULT_SOURCE instead of _BSD_SOURCE

Fixes a warning reported by gcc.

Gbp-Dch: ignore

apt-pkg/contrib/mmap.cc

@@ -16,7 +16,7 @@
    ##################################################################### */
 									/*}}}*/
 // Include Files							/*{{{*/
-#define _BSD_SOURCE
+#define _DEFAULT_SOURCE
 #include <config.h>
 
 #include <apt-pkg/mmap.h>