ANDROID: staging: ion: Fix missing entry in kerneldoc header

The kerneldoc header for struct ion_buffer was missing an
entry for @node which does exist in the rebase version of
android-mainline.

Change-Id: I230b90a2a38bbaf9af39ac0c03af55e2bdf2e133
Signed-off-by: Todd Kjos <tkjos@google.com>