sparc: fix sparc64 build due to leon.h inclusion

Stephen Rothwell <sfr@canb.auug.org.au> reported following error:
In file included from arch/sparc/kernel/prom_common.c:26:0:
arch/sparc/include/asm/leon.h:221:9: error: unknown type name 'irq_flow_handler_t'
arch/sparc/include/asm/leon.h:224:10: error: unknown type name 'irq_flow_handler_t'

Fix this by:
1) Avoid including leon.h in prom_commen.h (not needed)
2) Include irq.h in leon.h to avoid the missing symbol error

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>