tty: Add export.h for EXPORT_SYMBOL/THIS_MODULE to exporters

With module.h being implicitly everywhere via device.h, the absence
of explicitly including something for EXPORT_SYMBOL went unnoticed.
Since we are heading to fix things up and clean module.h from the
device.h file, we need to explicitly include these files now.

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
diff --git a/drivers/tty/serial/nwpserial.c b/drivers/tty/serial/nwpserial.c
index 9beaff1..dd4c31d 100644
--- a/drivers/tty/serial/nwpserial.c
+++ b/drivers/tty/serial/nwpserial.c
@@ -10,6 +10,7 @@
  *
  */
 #include <linux/init.h>
+#include <linux/export.h>
 #include <linux/console.h>
 #include <linux/serial.h>
 #include <linux/serial_reg.h>