Forward-declare struct private_handle_t in public header

Headers should include or forward-declare types that they are using.
This breaks a build failure in hardware/qcom/media due to the unknown
type.

Issue: FP2P-432
Issue: FP2A10-47
Change-Id: Ic0ed7957f272fa6983ca0a1d19a31bc2d9eebbfb
(cherry picked from commit 80722eb1f7c1f6693fd6d71cfd3d2e9c72e4ea1a)
1 file changed