lttng-babeltrace: add missing select BR2_PACKAGE_POPT

lttng-babeltrace unconditionally uses "popt" in its <pkg>_DEPENDENCIES
variable, but does not select it. This commit fixes this
inconsistency.

Fixes:

  http://autobuild.buildroot.org/results/274/2748a02b090ce078fc880bd821df3c913b8c8dfd/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>