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_buffer.c b/drivers/gpu/drm/drm_buffer.c
index 529a0db..08ccefe 100644
--- a/drivers/gpu/drm/drm_buffer.c
+++ b/drivers/gpu/drm/drm_buffer.c
@@ -32,6 +32,7 @@
  * Pauli Nieminen <suokkos-at-gmail-dot-com>
  */
 
+#include <linux/export.h>
 #include "drm_buffer.h"
 
 /**