fix: Static build fails with ctfser missing symbols

Spliting the ctf-writer from the main library had the side effect of not
including the ctfser convience library in the main lib anymore. Thus it
should now be linked with the binairies to satisfy the dependencies when
we statically link the plugins in.

Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Change-Id: I4626dcabce1914f0af2287afe8c3fcf7b7db9946
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1421
CI-Build: Philippe Proulx <eeppeliteloop@gmail.com>
Tested-by: jenkins
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>