staging: ion: export ion_free() for ion_heaps.

ion_free() is a generic function needed to release
an ion buffer back to the heap. So, export it for the
heap module.

Bug: 140294230
Test: ion-unit-tests

Change-Id: Iaa467ab3df18fa219412dd9f5913cd022a93455d
Signed-off-by: Sandeep Patil <sspatil@google.com>