ANDROID: GKI: signal: Export for __lock_task_sighand

    - Add EXPORT_SYMBOL_GPL for __lock_task_sighand()

    - This API is required by loadable driver module from samsung to
      Lock task structure, when reading task/process related information.

Signed-off-by: Abhilasha Rao <abhilasha.hv@samsung.corp-partner.google.com>
Bug: 158067689
Change-Id: Ia54e17b1f2630bcc648e7840cf36e743f08ba9c9
(cherry picked from commit a719abf03114cfc0b9f68937d9cd900d74d65cae)