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>