sips: fix memory leak

This patch fixes a memory leak in SipsIceTransport class.
gnutls_priority_init() allocates memory, no gnutls_priority_deinit()
is called to free it.
This last has been added in SipsIceTransport destructor.
This patch also fixes code rules error on priority_cache_ naming
(this is the variable initialized by gnutls_priority_init).

Change-Id: I32a8a7347d4af96886a0def2d9a6d77fe0fad7c7
Tuleap: #92