i965/program_cache: Cast the key to char * before adding key_size

i965/program_cache: Cast the key to char * before adding key_size

We're about to change the type of key to be brw_base_prog_key and that
will mean blindly adding the key size without a cast will lead to the
wrong calculation.  It's safer to cast to char * first anyway.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>