perf cs-etm: Add missing variable in cs_etm__process_queues()

Commit 5afd032961e8 "perf cs-etm: Don't flush when packet_queue fills
up" uses i as a loop counter in cs_etm__process_queues().  It was
backported to the 5.4 and 5.10 stable branches, but the i variable
doesn't exist there as it was only added in 5.15.

Declare i with the expected type.

Fixes: 1ed16732 ("perf cs-etm: Don't flush when packet_queue fills up")
Fixes: 26db806fa23e ("perf cs-etm: Don't flush when packet_queue fills up")
Signed-off-by: Ben Hutchings <benh@debian.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>