powerpc/mm: Fix build break with BOOK3S_64=n and MEMORY_HOTPLUG=y

The recently merged HPT (Hash Page Table) resize support broke the build
when BOOK3S_64=n (ie. 32-bit or 64-bit Book3E) and MEMORY_HOTPLUG=y:

  arch/powerpc/mm/mem.o: In function `.arch_add_memory':
  (.text+0x4e4): undefined reference to `.resize_hpt_for_hotplug'

Fix it by adding a dummy version.

Fixes: 438cc81a ("powerpc/pseries: Automatically resize HPT for memory hot add/remove")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/include/asm/sparsemem.h

@@ -16,9 +16,15 @@
 #endif /* CONFIG_SPARSEMEM */
 
 #ifdef CONFIG_MEMORY_HOTPLUG
-extern void resize_hpt_for_hotplug(unsigned long new_mem_size);
 extern int create_section_mapping(unsigned long start, unsigned long end);
 extern int remove_section_mapping(unsigned long start, unsigned long end);
+
+#ifdef CONFIG_PPC_BOOK3S_64
+extern void resize_hpt_for_hotplug(unsigned long new_mem_size);
+#else
+static inline void resize_hpt_for_hotplug(unsigned long new_mem_size) { }
+#endif
+
 #ifdef CONFIG_NUMA
 extern int hot_add_scn_to_nid(unsigned long scn_addr);
 #else