drm/sched: struct completion requires linux/completion.h inclusion

Fixes: 83a7772b ("drm/sched: Use completion to wait for sched->thread idle v2.")
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

include/drm/gpu_scheduler.h

@@ -26,6 +26,7 @@
 
 #include <drm/spsc_queue.h>
 #include <linux/dma-fence.h>
+#include <linux/completion.h>
 
 #define MAX_WAIT_SCHED_ENTITY_Q_EMPTY msecs_to_jiffies(1000)