aux_source_directory: Sort results to make it deterministic

The change in commit v3.6.0-rc1~54^2 (file: Sort GLOB results to make it
deterministic, 2016-05-14) makes sense for `aux_source_directory` too.

Signed-off-by: Junghyun Kim <jh0822.kim@samsung.com>
1 job for sort_auxsourcedirectory
Status Job ID Name Coverage
  External
passed kwrobot-commit-check