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>