include: Add va_copy macro if MSVC doesn't provide it

Signed-off-by: Andreas Schneider <asn@cryptomilk.org>
diff --git a/include/cmocka_private.h b/include/cmocka_private.h
index 454baf1..c67f8db 100644
--- a/include/cmocka_private.h
+++ b/include/cmocka_private.h
@@ -30,6 +30,10 @@
 #  undef inline
 #  define inline __inline
 
+#  ifndef va_copy
+#   define va_copy(dest, src) (dest = src)
+#  endif
+
 #  define strcasecmp _stricmp
 #  define strncasecmp _strnicmp