Fix: duplicate symbol with clang linker

Fix: duplicate symbol with clang linker

On MacOs with the clang linker, the build fails with:

 duplicate symbol _bt_ctf_value_null in:
     trace-ir/.libs/libtrace-ir.a(trace.o)
     trace-ir/.libs/libtrace-ir.a(trace-class.o)

Make the pointers extern.

Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Change-Id: I679dc5b647de235a19a958331f7f975c0cb0a08f
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1267
Reviewed-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>