Fix: flt.lttng-utils.debug-info: ".debug" extension not appended

Fix: flt.lttng-utils.debug-info: ".debug" extension not appended

Issue
=====
The g_strconcat() function returns a newly allocated string and does not
change the string in place as the code seems to assume. This results in
the call to g_strconcat() having no effects. Because of this, the
".debug" extension is not appended to the file name resulting in a
failure find the file on the file system.

Ignoring the return value of this call also results in the leaking of
the resulting char array.

Solution
========
Use g_snprintf() instead to concatenate the file name and its extension.

Known drawbacks
===============
None.

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>