powerpc/pseries: Fix build break when MEMORY_HOTREMOVE=n

We broke the build when CONFIG_MEMORY_HOTREMOVE=n:

  arch/powerpc/platforms/pseries/hotplug-memory.c:821:8: error: implicit
  declaration of function 'dlpar_memory_readd_by_index'

Add a dummy to fix it.

Fixes: e70d59700fc3 ("powerpc/pseries: Introduce memory hotplug READD operation")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/platforms/pseries/hotplug-memory.c

@@ -628,7 +628,10 @@ static int dlpar_memory_remove_by_index(u32 drc_index, struct property *prop)
 {
 	return -EOPNOTSUPP;
 }
-
+static int dlpar_memory_readd_by_index(u32 drc_index, struct property *prop)
+{
+	return -EOPNOTSUPP;
+}
 #endif /* CONFIG_MEMORY_HOTREMOVE */
 
 static int dlpar_add_lmb(struct of_drconf_cell *lmb)