trace: fix new gcc6 warnings

trace: fix new gcc6 warnings

src/gallium/drivers/trace/tr_context.c:1713:39: warning: ‘rbug_blocker_flags’ defined but not used [-Wunused-const-variable]
 static const struct debug_named_value rbug_blocker_flags[] = {
                                       ^~~~~~~~~~~~~~~~~~

Note that use of rbug_blocker_flags was removed in:

commit 5494332128da0b2826e85df5eeaa878bb5c30a4e
Author: Jakob Bornecrantz <jakob@vmware.com>
Date:   Wed May 12 19:26:19 2010 +0100

    trace: Remove rbug from trace

Signed-off-by: Rob Clark <robdclark@gmail.com>