kernel-netlink: Rename algorithm identifier from cast128 to cast5

Even if the XFRM identifier was named cast128 in the kernel before 2.6.31, it
actually never worked, because there is no such crypto algorithm.

The identifier has been changed to cast5 in
https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=245acb87
to make it work, so we should use that.

Fixes #633.

src/libhydra/plugins/kernel_netlink/kernel_netlink_ipsec.c

@@ -177,7 +177,7 @@ static kernel_algorithm_t encryption_algs[] = {
 	{ENCR_3DES,					"des3_ede"			},
 /*	{ENCR_RC5,					"***"				}, */
 /*	{ENCR_IDEA,					"***"				}, */
-	{ENCR_CAST,					"cast128"			},
+	{ENCR_CAST,					"cast5"				},
 	{ENCR_BLOWFISH,				"blowfish"			},
 /*	{ENCR_3IDEA,				"***"				}, */
 /*	{ENCR_DES_IV32,				"***"				}, */