file.c: with "sounds_search_custom_dir = yes", search "custom" directory

file.c: with "sounds_search_custom_dir = yes", search "custom" directory

With `sounds_search_custom_dir = yes`, we are supposed to search for sounds
in the `AST_DATA_DIR/sounds/custom` directory before searching the normal
directories.  Unfortunately, a recent change
(https://github.com/asterisk/asterisk/pull/1172) had a typo resulting in
the "custom" directory not being searched.  This change restores this
expected behavior.

Resolves: #1353
(cherry picked from commit 3ea4289a9a6504c0482d2fc0f50df5effd367ede)