[libxml2] Fix Windows compiler warning in testC14N.c

[Nick Wellnhofer <nwellnhof src gnome org>]

commit 619534ef2ed501e5206915064a5bc36153a897f7
Author: Nick Wellnhofer <wellnhofer aevum de>
Date:   Tue Jan 23 17:36:23 2018 +0100

    Fix Windows compiler warning in testC14N.c

 testC14N.c | 3 +++
 1 file changed, 3 insertions(+)
---
diff --git a/testC14N.c b/testC14N.c
index 86e58f27..addc800e 100644
--- a/testC14N.c
+++ b/testC14N.c
@@ -18,6 +18,9 @@
 #define STDOUT_FILENO fileno(stdout)
 #endif /* HAVE_UNISTD_H */
 #endif
+#ifdef _WIN32
+#include <io.h>
+#endif
 #ifdef HAVE_STDLIB_H
 #include <stdlib.h>
 #endif