imap-send.c: fix compiler warnings for OpenSSL 1.0

imap-send.c: fix compiler warnings for OpenSSL 1.0

The openssl/CHANGES file says:

    Let the TLSv1_method() etc. functions return a 'const' SSL_METHOD
    pointer and make the SSL_METHOD parameter in SSL_CTX_new,
    SSL_CTX_set_ssl_version and SSL_set_ssl_method 'const'.

In older versions, unqualified pointers were used, so we unfortunately
cannot unconditionally update the type of the variable we use.

Signed-off-by: Vietor Liu <vietor@vxwo.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>