Move utils_expand_path and utils_expand_path_keep_symlink to libpath.la

Move the GPLv2 helper functions utils_expand_path and
utils_expand_path_keep_symlink to libpath.la. This will allow utils.cpp
to be relicensed to LGPLv2.1 by making sure EfficiOS owns the copyright
for the entire source file.

Statically include libpath.la into libcommon-gpl.la.

The "lttng" executable is GPLv2 and only depends on libcommon-lgpl.la,
so it needs to explicitly list libpath.la as its dependency.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I2108e0bd35ce75797d4a20e65891c820a1dd79f8