[PATCH] tools/libxenstat: ensure strnlen() declaration is visible

[Jan Beulich <jbeulich@xxxxxxxx>]

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: 40fe714ca424 ("tools/libs/stat: use memcpy instead of strncpy in 
getBridge")
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/tools/libs/stat/xenstat_linux.c
+++ b/tools/libs/stat/xenstat_linux.c
@@ -20,6 +20,7 @@
  * Use is subject to license terms.
  */
 
+#define _GNU_SOURCE
 #include <fcntl.h>
 #include <dirent.h>
 #include <sys/types.h>