drm/amdgpu/vcn: include header for vcn_dec_sw_ring_emit_fence

Fixed warning: no previous prototype for 'vcn_dec_sw_ring_emit_fence'.

v2: regenerate patch after git rebase.
v3: update commit message.

Signed-off-by: James Zhu <James.Zhu@amd.com>
Reported-by: kernel test robot <lkp@intel.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>