Rename libcommon.so to libcommon-gpl.so

Rename libcommon.so to libcommon-gpl.so

libcommon is a static library is currently used by both liblttng-ctl
(LGPLv2.1) and all lttng-tools executables (GPLv2).

Given that some code in libcommon depends on liburcu, this introduces an
indirect dependency from liblttng-ctl to liburcu, which is unwanted.

This first step renames libcommon.so to libcommon-gpl.so. Following
steps will introduce a more lightweight libcommon-lgpl.so which only
contains LGPLv2.1 code, and removes the dependency on liburcu.

Backport Notes
--------------

ini_config has to link against liblttng-ctl since since the internal
configuration library is not split into ini-config and libconfig
(see 3299fd310).

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: Ia8f37ea229f68550200cbb1528216a505bbbd45f