tools-common.c: Fix resource leak in cgroup_string_list_add_directory()

tools-common.c: Fix resource leak in cgroup_string_list_add_directory()

Fix a Coverity warning about resource leak:

CID 465889: (#4 of 4): Resource leak (RESOURCE_LEAK)
32. leaked_storage: Variable fullpath going out of scope leaks the
storage it points to.

Fix it by releasing the 'fullpath', when file name does not matches
'*.conf' code path(s).

Signed-off-by: Kamalesh Babulal <kamalesh.babulal@oracle.com>
Signed-off-by: Tom Hromatka <tom.hromatka@oracle.com>