BUILD/MINOR: ssl: de-constify "ciphers" to avoid a warning on openssl-0.9.8

BUILD/MINOR: ssl: de-constify "ciphers" to avoid a warning on openssl-0.9.8

When building on openssl-0.9.8, since commit 23d5d37 ("MINOR: ssl: use
SSL_get_ciphers() instead of directly accessing the cipher list.") we get
the following warning :

	src/ssl_sock.c: In function 'ssl_sock_prepare_ctx':
	src/ssl_sock.c:1592: warning: passing argument 1 of 'SSL_CIPHER_description' discards qualifiers from pointer target type

This is because the openssl API has changed between 0.9.8 and 1.0.1 :

	0.9.8: char *SSL_CIPHER_description(SSL_CIPHER *,char *buf,int size);
	1.0.1: char *SSL_CIPHER_description(const SSL_CIPHER *,char *buf,int size);

So let's remove the "const" type qualifier to satisfy both versions.
Note that the fix above was backported to 1.5, so this one should as well.