ANDROID: staging: ion: move definition of attachment

The ion_dma_buf_attachment definition that is connected
to attachment of ion_buffer is used by dma buf ops.
However this definition should be available for heap's
dma buf ops as well as for built-in dma buf ops.
Therefore we move this definition to the ion header.

Bug: 151780326

Signed-off-by: Hyesoo Yu <hyesoo.yu@samsung.com>
Change-Id: Ib3023e29f48b1e2502f8d750af6645f65f6eea40