sparc: Fix minor SPARC32 compile error

When CONFIG_PROC_FS is unset, include/linux/interrupt.h defines
init_irq_proc() as an empty function.

arch/sparc/kernel/irq_32.c defines this function unconditionally.

Fix the latter so that it only defines this function when CONFIG_PROC_FS
is set.

This fixes the following error:
arch/sparc/kernel/irq_32.c:672: error: redefinition of 'init_irq_proc'
include/linux/interrupt.h:461: error: previous definition of
'init_irq_proc' was here

This was found using randconfig builds.

Signed-off-by: Julian Calaby <julian.calaby@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>