powerpc: Add missing prototype for time_init()

The function time_init did not have a prototype defined in the time.h
header. Fix the following warning (treated as error in W=1):

arch/powerpc/kernel/time.c:1068:13: error: no previous prototype for ‘time_init’

Signed-off-by: Mathieu Malaterre <[e-mail]>
Signed-off-by: Michael Ellerman <[e-mail]>

arch/powerpc/include/asm/time.h

···
 DECLARE_PER_CPU(struct cpu_usage, cpu_usage_array);
 
 extern void secondary_cpu_time_init(void);
+extern void __init time_init(void);
 
 DECLARE_PER_CPU(u64, decrementers_next_tb);