drm/i915: add extern C guard for the UAPI header

Cc: Jani Nikula <jani.nikula@linux.intel.com>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Acked-by: Daniel Vetter <daniel.vetter@intel.com>

include/uapi/drm/i915_drm.h

@@ -29,6 +29,10 @@
 
 #include "drm.h"
 
+#if defined(__cplusplus)
+extern "C" {
+#endif
+
 /* Please note that modifications to all structs defined here are
  * subject to backwards-compatibility constraints.
  */
@@ -1170,4 +1174,8 @@ struct drm_i915_gem_context_param {
 	__u64 value;
 };
 
+#if defined(__cplusplus)
+}
+#endif
+
 #endif /* _UAPI_I915_DRM_H_ */