trace: fix generated code build break

If the QEMU source dir is

    /var/tmp/aaa-qemu-clone

and the build dir is

    /var/tmp/qemu-aio-poll-v2

Then I get an error as:

trace/generated-tracers.c:15950:13: error: invalid suffix "_trace_events"
 on integer constant
 TraceEvent *2_trace_events[] = {
             ^
trace/generated-tracers.c:15950:13: error: expected identifier or ‘(’ before
 numeric constant
trace/generated-tracers.c: In function ‘trace_2_register_events’:
trace/generated-tracers.c:17949:32: error: invalid suffix "_trace_events" on
 integer constant
     trace_event_register_group(2_trace_events);
                                ^
make: *** [trace/generated-tracers.o] Error 1

This patch fixes the issue.

Reported-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Greg Kurz <groug@kaod.org>
Tested-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>