Make _utils_expand_path static

Make _utils_expand_path static

It's only used in its own file, so make it static.

Fixes:

      CC       utils.lo
    /home/smarchi/src/lttng-tools/src/common/utils.c:328:7: error: no previous declaration for ‘_utils_expand_path’ [-Werror=missing-declarations]
     char *_utils_expand_path(const char *path, bool keep_symlink)
           ^~~~~~~~~~~~~~~~~~

Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Change-Id: Ibab3bf9994f29b1a841e9d3ddc9bfce1f8e73032
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>