Fix the following warning:

  CC      arch/mips/ralink/irq-gic.o
arch/mips/ralink/irq-gic.c:15:5: warning: no previous prototype for 'get_c0_perfcount_int' [-Wmissing-prototypes]
   15 | int get_c0_perfcount_int(void)
      |     ^~~~~~~~~~~~~~~~~~~~
Signed-off-by: Vincent Legoll <vincent.legoll@gmail.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>