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

Fix the forward declaration.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: NChris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: NJani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191219155652.2666-1-jani.nikula@intel.com

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

@@ -7,7 +7,7 @@
 #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);