ARM: opcodes: add missing include of linux/linkage.h

opcodes.h wants to declare an asmlinkage function, so we need to include
linux/linkage.h

Acked-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Dave Martin <dave.martin@linaro.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>