riscv: time: Fix no prototype for time_init

Fix the following W=1 compilation warning:
arch/riscv/kernel/time.c:16:13: warning: no previous prototype for ‘time_init’ [-Wmissing-prototypes]
   16 | void __init time_init(void)
      |             ^~~~~~~~~

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Nanyong Sun <sunnanyong@huawei.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>

arch/riscv/include/asm/timex.h

@@ -88,4 +88,6 @@ static inline int read_current_timer(unsigned long *timer_val)
 	return 0;
 }
 
+extern void time_init(void);
+
 #endif /* _ASM_RISCV_TIMEX_H */

arch/riscv/kernel/time.c

@@ -9,6 +9,7 @@
 #include <linux/delay.h>
 #include <asm/sbi.h>
 #include <asm/processor.h>
+#include <asm/timex.h>
 
 unsigned long riscv_timebase;
 EXPORT_SYMBOL_GPL(riscv_timebase);