radv: Fix driver UUID SHA1 init.

Was missing the init, found by Emil.

Fixes: d17443a4 "radv: Use build ID if available for cache UUID."
CC: <mesa-stable@lists.freedesktop.org>
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
(cherry picked from commit 0a77e70d)