msm: secure_buffer: Add missing include

secure_buffer.h makes use of the scatterlist structure, but doesn't
include the header file that defines it.  Until now we've been getting
lucky that all users of secure_buffer.h have been including the
scatterlist header file themselves, but this might not always be the
case.  Add the missing include.

Change-Id: If7598c26a1ef3551132c6eb62ac9bd6d447654d0
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>