ASoC: wm_adsp_fw_find_test: Fix missing NULL terminator on file name list

The array wm_adsp_fw_find_test_dir_all_files[] needs a NULL terminator.

It was documented at line 41 that dir_files points to a NULL-terminated
array.

/* NULL-terminated array of pointers to filenames ... <snip> */
const char * const *dir_files;

This fixes the error reported by KASAN:

[17:46:43] BUG: KASAN: global-out-of-bounds in
wm_adsp_fw_find_test_pick_file+0x8cc/0xc80
[17:46:43] Read of size 8 at addr ffff800081e5ffe8 by task
kunit_try_catch/18349

Signed-off-by: Richard Fitzgerald <rf@opensource.cirrus.com>
Fixes: bf2d44d07de7 ("ASoC: wm_adsp: Add kunit test for firmware file search")
Link: https://patch.msgid.link/20260326100853.1582886-1-rf@opensource.cirrus.com
Signed-off-by: Mark Brown <broonie@kernel.org>