configure: Fix compile warning in utimensat/futimens test

Fix compile warning in the utimensat/futimens test ("implicit
declaration of function 'utimensat'", ditto futimens) by
adding a missing include.
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
Reviewed-by: NStefan Weil <sw@weilnetz.de>
Signed-off-by: NBlue Swirl <blauwirbel@gmail.com>

configure

@@ -2343,6 +2343,7 @@ cat > $TMPC << EOF
 #define _ATFILE_SOURCE
 #include <stddef.h>
 #include <fcntl.h>
+#include <sys/stat.h>
 
 int main(void)
 {