drm/i915: make intel_guc_ct.h self-contained

Add the minimal includes/declarations to make the header self-contained,
and ensure it stays that way.
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190626144020.2155-9-jani.nikula@intel.com

drivers/gpu/drm/i915/Makefile.header-test

@@ -19,6 +19,7 @@ header_test := \
 	i915_vgpu.h \
 	intel_csr.h \
 	intel_drv.h \
+	intel_guc_ct.h \
 	intel_pm.h \
 	intel_runtime_pm.h \
 	intel_sideband.h \

drivers/gpu/drm/i915/intel_guc_ct.h

@@ -24,11 +24,14 @@
 #ifndef _INTEL_GUC_CT_H_
 #define _INTEL_GUC_CT_H_
 
-struct intel_guc;
-struct i915_vma;
+#include <linux/spinlock.h>
+#include <linux/workqueue.h>
 
 #include "intel_guc_fwif.h"
 
+struct i915_vma;
+struct intel_guc;
+
 /**
  * DOC: Command Transport (CT).
  *