powerpc: Add missing prototype for hdec_interrupt

In commit dabe859e ("powerpc: Give hypervisor decrementer interrupts
their own handler") an empty body function was added, but no prototype
was declared. Fix warning (treated as error in W=1):

  arch/powerpc/kernel/time.c:629:6: error: no previous prototype for ‘hdec_interrupt’
Signed-off-by: Mathieu Malaterre <malat@debian.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/include/asm/time.h

@@ -31,6 +31,7 @@ extern void to_tm(int tim, struct rtc_time * tm);
 extern void tick_broadcast_ipi_handler(void);
 
 extern void generic_calibrate_decr(void);
+extern void hdec_interrupt(struct pt_regs *regs);
 
 /* Some sane defaults: 125 MHz timebase, 1GHz processor */
 extern unsigned long ppc_proc_freq;