build fixes for scripts/kconfig

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/scripts/kconfig/conf.c b/scripts/kconfig/conf.c
index 9befa2b..6e09788 100644
--- a/scripts/kconfig/conf.c
+++ b/scripts/kconfig/conf.c
@@ -3,6 +3,8 @@
  * Released under the terms of the GNU GPL v2.0.
  */
 
+#define _XOPEN_SOURCE 700
+
 #include <ctype.h>
 #include <stdlib.h>
 #include <stdio.h>