drm/i915: make i915_globals.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/20190626144020.2155-6-jani.nikula@intel.com

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

@@ -8,6 +8,7 @@ header_test := \
 	i915_drv.h \
 	i915_fixed.h \
 	i915_gem_gtt.h \
+	i915_globals.h \
 	i915_irq.h \
 	i915_params.h \
 	i915_priolist_types.h \

drivers/gpu/drm/i915/i915_globals.h

@@ -7,6 +7,8 @@
 #ifndef _I915_GLOBALS_H_
 #define _I915_GLOBALS_H_
 
+#include <linux/types.h>
+
 typedef void (*i915_global_func_t)(void);
 
 struct i915_global {