MIPS: ralink: Fix missing `get_c0_perfcount_int` prototype

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>

arch/mips/ralink/irq-gic.c

@@ -10,6 +10,7 @@
 #include <linux/of.h>
 #include <linux/irqchip.h>
 #include <asm/mips-cps.h>
+#include <asm/time.h>
 
 int get_c0_perfcount_int(void)
 {