staging: ion: Build fix for compat_ion.c

compat_get_ion_handle_data is missing a declaration for
the return value.

This patch simply adds it, so things build.

Change-Id: I1a72a3c56975dc614322a63852f2a6554f2be107
Signed-off-by: John Stultz <john.stultz@linaro.org>
Git-commit: d4e564429cb89a90b9d5482af884b0b6a85aa284
Git-repo: https://android.googlesource.com/kernel/common/


Signed-off-by: Laura Abbott <lauraa@codeaurora.org>