dma-fence: Make symbol 'dma_fence_lockdep_map' static

The sparse tool complains as follows:

drivers/dma-buf/dma-fence.c:249:25: warning:
symbol 'dma_fence_lockdep_map' was not declared. Should it be static?

This variable is not used outside of dma-fence.c, so this commit
marks it static.

Fixes: 5fbff813 ("dma-fence: basic lockdep annotations")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>

Link: https://patchwork.freedesktop.org/patch/msgid/dc5e3b19-2087-44ab-a28c-ddb38ff8861a@email.android.com