printk: export console_drivers

this symbol is needed by drivers/video/xen-fbfront.ko.

[ cherry-picked from tip/core/printk ]
Signed-off-by: NIngo Molnar <mingo@elte.hu>

kernel/printk.c

@@ -75,6 +75,8 @@ EXPORT_SYMBOL(oops_in_progress);
 static DECLARE_MUTEX(console_sem);
 static DECLARE_MUTEX(secondary_console_sem);
 struct console *console_drivers;
+EXPORT_SYMBOL_GPL(console_drivers);
+
 /*
  * This is used for debugging the mess that is the VT code by
  * keeping track if we have the console semaphore held. It's