genirq: Make GENERIC_IRQ_MULTI_HANDLER depend on !MULTI_IRQ_HANDLER

These config switches enable the same code in the core and the not yet
converted architecture code. They can be selected both by randconfig builds
and cause linker error because the same symbols are defined twice.

Make the new GENERIC_IRQ_MULTI_HANDLER depend on !MULTI_IRQ_HANDLER to
prevent that. The dependency will be removed once all architectures are
converted over.

Signed-off-by: Palmer Dabbelt <palmer@sifive.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Link: https://lkml.kernel.org/r/20180404043130.31277-4-palmer@sifive.com