gpu: Add export.h as required to drivers/gpu files.

They need this to get all the EXPORT_SYMBOL variants and THIS_MODULE

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
diff --git a/drivers/gpu/drm/drm_usb.c b/drivers/gpu/drm/drm_usb.c
index 206d230..445003f 100644
--- a/drivers/gpu/drm/drm_usb.c
+++ b/drivers/gpu/drm/drm_usb.c
@@ -1,5 +1,6 @@
 #include "drmP.h"
 #include <linux/usb.h>
+#include <linux/export.h>
 
 #ifdef CONFIG_USB
 int drm_get_usb_dev(struct usb_interface *interface,