include: Add stdarg.h so we can check for va_copy macro

Signed-off-by: Andreas Schneider <asn@cryptomilk.org>

include/libssh/priv.h

@@ -60,6 +60,7 @@
 
 # ifdef _MSC_VER
 #  include <stdio.h>
+#  include <stdarg.h> /* va_copy define check */
 
 /* On Microsoft compilers define inline to __inline on all others use inline */
 #  undef inline