common/ini-config: Re-license to LGPL-2.1-only

common/ini-config: Re-license to LGPL-2.1-only

Observed issue
==============

For ABI compatibility, the symbols e.g., `config_str_yes`, will need
to be included in liblttng-ctl. As the library is LGPL licensed, the
GPL code shouldn't be linked to it.

Solution
========

Re-license to LGPL-2.1-only.

Known drawbacks
===============

None.

Change-Id: Ia8163522e5dc21d44af98b03c019298019e3f05a
Signed-off-by: Kienan Stewart <kstewart@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>