tooling/headers: Sync the tools/include/uapi/drm/i915_drm.h UAPI header

Last minute upstream update to one of the UAPI headers - sync it with tooling,
to address this warning:

  Warning: Kernel ABI header at 'tools/include/uapi/drm/i915_drm.h' differs from latest version at 'include/uapi/drm/i915_drm.h'

Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

Author: ingomolnar

tools/include/uapi/drm/i915_drm.h

@@ -829,6 +829,7 @@ struct drm_i915_gem_exec_fence {
 
 #define I915_EXEC_FENCE_WAIT            (1<<0)
 #define I915_EXEC_FENCE_SIGNAL          (1<<1)
+#define __I915_EXEC_FENCE_UNKNOWN_FLAGS (-(I915_EXEC_FENCE_SIGNAL << 1))
 	__u32 flags;
 };