xtensa: fix build error in xtensa/include/asm/io.h

Caused by commit 6c03438edeb5c359af35f060ea016ca65671c269

kernel.h: doesn't explicitly use bug.h, so don't include it.

This header uses bug.h so explicitly include it now that the implicit presence was removed by 6c03438ed.

index d04cd3a..4beb43c 100644
@@ -14,6 +14,7 @@
 #ifdef __KERNEL__
 #include <asm/byteorder.h>
 #include <asm/page.h>
+#include <linux/bug.h>
 #include <linux/kernel.h>
 
 #include <linux/types.h>

1.7.9.1