cygwin: add missing stdarg.h include

Further header file standarization in Cygwin uncovered a lazy
indirect include in bsd-cygwin_util.c

Signed-off-by: Corinna Vinschen <vinschen@redhat.com>
diff --git a/openbsd-compat/bsd-cygwin_util.c b/openbsd-compat/bsd-cygwin_util.c
index 398a5f6..fb49e30 100644
--- a/openbsd-compat/bsd-cygwin_util.c
+++ b/openbsd-compat/bsd-cygwin_util.c
@@ -36,6 +36,7 @@
 #include <fcntl.h>
 #include <string.h>
 #include <unistd.h>
+#include <stdarg.h>
 
 #include "xmalloc.h"