msm: secure_buffer: fix stub functions

All of these need to be 'static inline' to avoid multiple
definition problems when MSM_SECURE_BUFFER=n.

Change-Id: Ie2f9e6204964e46f837ae1d77a10fb81de48671a
Signed-off-by: Jeremy Gebben <jgebben@codeaurora.org>
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>