lttng.ust: Do not skip unknown lines in addr2line output

lttng.ust: Do not skip unknown lines in addr2line output

addr2line will use "??" to indicate unknown function names or
source files. In these cases we should still report the other
information that may be available, instead of skipping the whole
callsite.

Change-Id: I2cd055b68950c8a6b5c21fd984cd82f2e3544245
Signed-off-by: Alexandre Montplaisir <alexmonthy@efficios.com>
Reviewed-on: https://git.eclipse.org/r/77327
Reviewed-by: Hudson CI
Reviewed-by: Matthew Khouzam <matthew.khouzam@ericsson.com>