powerpc/smp: Move setup_profiling_timer() under CONFIG_PROFILING

setup_profiling_timer() is only needed when CONFIG_PROFILING is enabled.

Fixes the following W=1 warning when CONFIG_PROFILING=n:
  linux/arch/powerpc/kernel/smp.c:1638:5: error: no previous prototype for ‘setup_profiling_timer’
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20211124093254.1054750-5-mpe@ellerman.id.au

arch/powerpc/kernel/smp.c

@@ -1635,10 +1635,12 @@ void start_secondary(void *unused)
 	BUG();
 }
 
+#ifdef CONFIG_PROFILING
 int setup_profiling_timer(unsigned int multiplier)
 {
 	return 0;
 }
+#endif
 
 static void fixup_topology(void)
 {