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_platform.c b/drivers/gpu/drm/drm_platform.c
index 2a8b626..ae9db5e 100644
--- a/drivers/gpu/drm/drm_platform.c
+++ b/drivers/gpu/drm/drm_platform.c
@@ -25,6 +25,7 @@
  * WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
  */
 
+#include <linux/export.h>
 #include "drmP.h"
 
 /**