openrisc: add missing header inclusion

Prevents build issue with updated toolchain
Reported-by: Jack Thomasson <jkt@moonlitsw.com>
Tested-by: Christian Svensson <blue@cmd.nu>
Signed-off-by: Stefan Kristiansson <stefan.kristiansson@saunalahti.fi>
Signed-off-by: Jonas Bonn <jonas@southpole.se>

arch/openrisc/kernel/head.S

@@ -19,6 +19,7 @@
 #include <linux/threads.h>
 #include <linux/errno.h>
 #include <linux/init.h>
+#include <linux/serial_reg.h>
 #include <asm/processor.h>
 #include <asm/page.h>
 #include <asm/mmu.h>