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.
diff --git a/src/ssl_sock.c b/src/ssl_sock.c
index 4e39c8b..fa98a6b 100644
--- a/src/ssl_sock.c
+++ b/src/ssl_sock.c
@@ -1479,7 +1479,7 @@
 		SSL_MODE_ACCEPT_MOVING_WRITE_BUFFER |
 		SSL_MODE_RELEASE_BUFFERS;
 	STACK_OF(SSL_CIPHER) * ciphers = NULL;
-	SSL_CIPHER const * cipher = NULL;
+	SSL_CIPHER * cipher = NULL;
 	char cipher_description[128];
 	/* The description of ciphers using an Ephemeral Diffie Hellman key exchange
 	   contains " Kx=DH " or " Kx=DH(". Beware of " Kx=DH/",