Define _DEFAULT_SOURCE instead of _BSD_SOURCE

Glibc feature macro _BSD_SOURCE is deprecated in favour of
_DEFAULT_SOURCE since glibc-2.20:

In file included from /usr/include/bits/libc-header-start.h:33,
                 from /usr/include/limits.h:26,
                 from /usr/lib/gcc/x86_64-redhat-linux/14/include/limits.h:210,
                 from /usr/lib/gcc/x86_64-redhat-linux/14/include/syslimits.h:7,
                 from /usr/lib/gcc/x86_64-redhat-linux/14/include/limits.h:34,
                 from /usr/lib64/glib-2.0/include/glibconfig.h:11,
                 from /usr/include/glib-2.0/glib/gtypes.h:34,
                 from /usr/include/glib-2.0/glib/galloca.h:34,
                 from /usr/include/glib-2.0/glib.h:32,
                 from /home/test/librepo/librepo/handle.c:24:
/usr/include/features.h:197:3: warning: #warning "_BSD_SOURCE and _SVID_SOURCE are deprecated, use _DEFAULT_SOURCE" [-Wcpp]
  197 | # warning "_BSD_SOURCE and _SVID_SOURCE are deprecated, use _DEFAULT_SOURCE"
      |   ^~~~~~~

This patch adds _DEFAULT_SOURCE definition to do it like
librepo/downloader.c aleady does. (Though replacing _BSD_SOURCE would
do it even on RHEL 8 where glibc is newer than 2.20.)

librepo/handle.c

@@ -20,6 +20,7 @@
 
 #define _POSIX_C_SOURCE 200809L
 #define _BSD_SOURCE
+#define _DEFAULT_SOURCE
 
 #include <glib.h>
 #include <errno.h>