openrisc: explicitly include linux/bug.h in asm/fixmap.h

openrisc's asm/fixmap.h uses the BUG() and BUG_ON() macros but relies on
implict inclusion of linux/bug.h which means that changes in other
headers could break the build. Thus, add an explicit include.
Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: Stafford Horne <shorne@gmail.com>

arch/openrisc/include/asm/fixmap.h

@@ -27,6 +27,7 @@
 #define FIXADDR_TOP	((unsigned long) (-2*PAGE_SIZE))
 
 #include <linux/kernel.h>
+#include <linux/bug.h>
 #include <asm/page.h>
 
 /*