restoring SSL_MODE_ACCEPT_MOVING_WRITE_BUFFER constant

This constant was removed in pyca@895de04 but it is still needed to deal with an issue in PyOpenSSL described here cherrypy/cheroot#245 and PR pyca/pyopenssl#1242.

Author: julianz-

src/_cffi_src/openssl/ssl.py

@@ -108,6 +108,7 @@
 static const long SSL_CB_HANDSHAKE_DONE;
 static const long SSL_MODE_RELEASE_BUFFERS;
 static const long SSL_MODE_ENABLE_PARTIAL_WRITE;
+static const long SSL_MODE_ACCEPT_MOVING_WRITE_BUFFER;
 static const long SSL_MODE_AUTO_RETRY;
 static const long TLS_ST_BEFORE;
 static const long TLS_ST_OK;