package: cfgfs: Fix for compiling with gcc-14

The asprintf() function is not exported unless _GNU_SOURCE has been
defined.

Signed-off-by: Phil Sutter <phil@nwl.cc>

package/cfgfs/src/ft_pack.c

@@ -21,6 +21,7 @@
  * the possibility of such damage or existence of a defect.
  */
 
+#define _GNU_SOURCE
 #include <sys/param.h>
 #include <sys/stat.h>
 #include <fcntl.h>