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_vm.c b/drivers/gpu/drm/drm_vm.c
index 5db96d45..8c03eaf 100644
--- a/drivers/gpu/drm/drm_vm.c
+++ b/drivers/gpu/drm/drm_vm.c
@@ -34,6 +34,7 @@
  */
 
 #include "drmP.h"
+#include <linux/export.h>
 #if defined(__ia64__)
 #include <linux/efi.h>
 #include <linux/slab.h>