tee: shm: make function __tee_shm_alloc static

The function __tee_shm_alloc is local to the source and does
not need to be in global scope, so make it static.

Cleans up sparse warning:
symbol '__tee_shm_alloc' was not declared. Should it be static?

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>