[PATCH] kconfig: ensure strndup() declaration is visible

[Jan Beulich]

Its guard was updated such that it is visible by default when POSIX 2008
was adopted by glibc. It's not visible by default on older glibc.

Fixes: f80fe2b34f08 ("xen: Update Kconfig to Linux v5.4")
Signed-off-by: Jan Beulich <jbeul...@suse.com>

--- a/xen/tools/kconfig/parser.y
+++ b/xen/tools/kconfig/parser.y
@@ -4,6 +4,7 @@
  */
 %{
 
+#define _GNU_SOURCE
 #include <ctype.h>
 #include <stdarg.h>
 #include <stdio.h>