IP27: Make declaration and definition of rt_timer_irq match.

Without, the LTO compiler will complain:

[...]
  LD      init/built-in.o
  LDFINAL vmlinux.o
arch/mips/sgi-ip27/ip27-irq.c:379:22: warning: type of ‘rt_timer_irq’ does not match original declaration [enabled by default]
  extern unsigned int rt_timer_irq;
                      ^
arch/mips/sgi-ip27/ip27-timer.c:72:5: note: previously declared here
 int rt_timer_irq;
     ^
  MODPOST vmlinux.o
[...]

And without LTO, it's still good stile to things match.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>