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 <malat@debian.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/include/asm/time.h

@@ -205,6 +205,7 @@ struct cpu_usage {
 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);