Add necessary #includes and forward declarations to make the header
compile on its own.

While at it, also remove a useless forward declaration.
Reviewed-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/46c2acf80f1d214182a96a97fa6322ab89a3de05.1607422863.git.jani.nikula@intel.com