ctf: fix parser not parsing "thing := keyword typealias;" regression

ctf: fix parser not parsing "thing := keyword typealias;" regression

This regression was introduced when dynamic scopes added stricter checks
to tsdl parsing. The checks are good, this fix will extract the pertinent
data before the check.

Change-Id: Id0e0571973569f219339b3733545286a0b53face
Signed-off-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Reviewed-on: https://git.eclipse.org/r/52447
Reviewed-by: Hudson CI
Reviewed-by: Bernd Hufmann <bernd.hufmann@ericsson.com>
Tested-by: Bernd Hufmann <bernd.hufmann@ericsson.com>