MIPS: ralink: Fix missing `plat_time_init` prototype

Fix the following warning:

CC arch/mips/ralink/timer-gic.o
arch/mips/ralink/timer-gic.c:18:13: warning: no previous prototype for 'plat_time_init' [-Wmissing-prototypes]
18 | void __init plat_time_init(void)
| ^~~~~~~~~~~~~~

Signed-off-by: Vincent Legoll <[e-mail]>
Signed-off-by: Thomas Bogendoerfer <[e-mail]>

arch/mips/ralink/timer-gic.c

···
 #include <linux/of_clk.h>
 #include <linux/clocksource.h>
 
+#include <asm/time.h>
+
 #include "common.h"
 
 void __init plat_time_init(void)