powerpc/irq: Drop forward declaration of struct irqaction

Since the commit identified below, the forward declaration of
struct irqaction is useless. Drop it.

Fixes: b709c083

 ("ppc64: move stack switching up in interrupt processing")
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/e0bcdabac45fcd26c02d7df273bd4a5827c6033d.1596716375.git.christophe.leroy@csgroup.eu