drm/amd/amdgpu/amdgpu_virt: Make local function 'amdgpu_virt_update_vf2pf_work_item()' static

Fixes the following W=1 kernel build warning(s):

drivers/gpu/drm/amd/amdgpu/amdgpu_virt.c:560:6: warning: no previous prototype for ‘amdgpu_virt_update_vf2pf_work_item’ [-Wmissing-prototypes]

Cc: Alex Deucher <[e-mail]>
Cc: "Christian König" <[e-mail]>
Cc: David Airlie <[e-mail]>
Cc: Daniel Vetter <[e-mail]>
Cc: [e-mail]
Cc: [e-mail]
Signed-off-by: Lee Jones <[e-mail]>
Signed-off-by: Alex Deucher <[e-mail]>

drivers/gpu/drm/amd/amdgpu/amdgpu_virt.c

···
 	return 0;
 }
 
-void amdgpu_virt_update_vf2pf_work_item(struct work_struct *work)
+static void amdgpu_virt_update_vf2pf_work_item(struct work_struct *work)
 {
 	struct amdgpu_device *adev = container_of(work, struct amdgpu_device, virt.vf2pf_work.work);