drm/vgem: Fix non static symbol warning

Fixes the following sparse warning:

drivers/gpu/drm/vgem/vgem_fence.c:75:24: warning:
  symbol 'vgem_fence_ops' was not declared. Should it be static?
Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1468932262-26554-1-git-send-email-weiyj_lk@163.com

drivers/gpu/drm/vgem/vgem_fence.c

@@ -72,7 +72,7 @@ static void vgem_fence_timeline_value_str(struct fence *fence, char *str,
 	snprintf(str, size, "%u", fence_is_signaled(fence) ? fence->seqno : 0);
 }
 
-const struct fence_ops vgem_fence_ops = {
+static const struct fence_ops vgem_fence_ops = {
 	.get_driver_name = vgem_fence_get_driver_name,
 	.get_timeline_name = vgem_fence_get_timeline_name,
 	.enable_signaling = vgem_fence_enable_signaling,