BACKPORT: 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?

Change-Id: I6bbd0f64c3a334f63ec6a834f43c3bc5830d7335
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>
(cherry picked from commit 80ec6f5de60b6934f145b2f7e5369592bcab85f3)
Signed-off-by: Victor Chong <victor.chong@linaro.org>