trace: fix tcg tracing build breakage

trace: fix tcg tracing build breakage

Commit 0ab8ed18a6fe98bfc82705b0f041fbf2a8ca5b60 ("trace: switch to
modular code generation for sub-directories") forgot to convert "tcg"
trace events to the modular code generation approach where each
sub-directory has its own trace-events file.

This patch fixes compilation for "tcg" trace events.  Currently they are
only used in the root ./trace-events file.

"tcg" trace events can only be used in the root ./trace-events file for
the time being.

Reported-by: Peter Maydell <[email protected]>
Tested-by: Emilio G. Cota <[email protected]>
Signed-off-by: Stefan Hajnoczi <[email protected]>
Message-id: 20170327131718[email protected]
Suggested-by: Emilio G. Cota <[email protected]>
Signed-off-by: Stefan Hajnoczi <[email protected]>