package/optee-client: fix gid_t build issue in OP-TEE 3.12 libckteec

package/optee-client: fix gid_t build issue in OP-TEE 3.12 libckteec

Include missing unistd.h to support type gid_t in ckteec library
of optee-client 3.12.0. This change fixes [1]. The fix has been
posted to optee-client forum, see [2].

Link: [1] http://autobuild.buildroot.net/results/34b9946e6d59112a7eead304933534ad4739a84c/build-end.log
Link: [2] https://github.com/OP-TEE/optee_client/pull/262

Signed-off-by: Etienne Carriere <etienne.carriere@linaro.org>
[yann.morin.1998@free.fr:
  - use an actual backport now that upstream applied it
  - fix typo in title
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>