powerpc: Move definitions of secondary CPU spinloop to header file

Those definitions are currently declared extern in the .c file where
they are used, move them to a header file instead.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/platforms/pseries/smp.c

@@ -56,8 +56,6 @@
  */
 static cpumask_t of_spin_map;
 
-extern void generic_secondary_smp_init(unsigned long);
-
 /**
  * smp_startup_cpu() - start the given cpu
  *