drm/msm: include linux/sched/task.h

Without this header file, compile-testing may run into a missing
declaration:

drivers/gpu/drm/msm/msm_gpu.c:444:4: error: implicit declaration of function 'put_task_struct' [-Werror,-Wimplicit-function-declaration]

Fixes: 482f9632 ("drm/msm: Fix task dump in gpu recovery")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Jordan Crouse <jcrouse@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>