drm/i915/selftests: make mock_context.h self-contained

Fix the forward declaration.

Cc: Chris Wilson <[e-mail]>
Cc: Masahiro Yamada <[e-mail]>
Reviewed-by: Chris Wilson <[e-mail]>
Signed-off-by: Jani Nikula <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/i915/gem/selftests/mock_context.h

···
 #ifndef __MOCK_CONTEXT_H
 #define __MOCK_CONTEXT_H
 
-struct drm_file;
+struct file;
 struct drm_i915_private;
 
 void mock_init_contexts(struct drm_i915_private *i915);