Fix an MSVC warning.

Reviewed-by: NKurt Roeckx <kurt@openssl.org>
Reviewed-by: NRich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/1168)

crypto/include/internal/cryptlib.h

@@ -16,6 +16,7 @@
 # include "e_os.h"
 
 # ifdef OPENSSL_USE_APPLINK
+#  undef BIO_FLAGS_UPLINK
 #  define BIO_FLAGS_UPLINK 0x8000
 #  include "ms/uplink.h"
 # endif