Fix custom platform abstraciton after merge

Change-Id: Ie5f634aeca7c9af3b0f5233c5c5084713e6ea8a9
audio: msm8660: Use msm_ion header

 * The ion.h header has long since removed MSM-specific bits.
 * Even ancient kernels should have support for at least a dummy
   msm_ion.h, so enable it for compatibility with new kernels.

Change-Id: Ie3381913c8ff8a8e61038d8962f1455131dc4659