include: Add support for older MSVC versions

Signed-off-by: Andreas Schneider <asn@cryptomilk.org>
(cherry picked from commit 30a7229fc5)

include/libssh/priv.h

@@ -65,6 +65,10 @@
 #  undef inline
 #  define inline __inline
 
+#  ifndef va_copy
+#   define va_copy(dest, src) (dest = src)
+#  endif
+
 #  define strcasecmp _stricmp
 #  define strncasecmp _strnicmp
 #  if ! defined(HAVE_ISBLANK)