tee-supplicant: fix build with kernel < 4.16

Commit 3ac968ee moved linux/tee.h from
libteec/include to libteec/src resulting in the following build failure
with any kernel < 4.16 (i.e before
https://github.com/torvalds/linux/commit/033ddf12bcf5326b93bd604f50a7474a434a35f9

):

/home/buildroot/autobuild/instance-3/output-1/build/optee-client-4.0.0/tee-supplicant/src/tee_supplicant.c: In function 'register_local_shm':
/home/buildroot/autobuild/instance-3/output-1/build/optee-client-4.0.0/tee-supplicant/src/tee_supplicant.c:356:44: error: storage size of 'data' isn't known
  356 |         struct tee_ioctl_shm_register_data data;
      |                                            ^~~~

To fix this build failure, update CMakeLists.txt and Makefile of
tee-supplicant to add libteec/src to the include directories.

Fixes: 3ac968ee ("Makefile, cmake: move teec related headers")

Acked-by: Jerome Forissier <jerome.forissier@linaro.org>
Acked-by: Jens Wiklander <jens.wiklander@linaro.org>
Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>