x86: 32-bit numa, build fix

on Summit it's possible to have:

 CONFIG_ACPI_SRAT=y
 CONFIG_HAVE_ARCH_PARSE_SRAT=y

in which case acpi.h defines the acpi_numa_slit_init() and
acpi_numa_processor_affinity_init() methods as a macro.

arch/x86/mm/discontig_32.c

@@ -455,7 +455,7 @@ int memory_add_physaddr_to_nid(u64 addr)
 EXPORT_SYMBOL_GPL(memory_add_physaddr_to_nid);
 #endif
 
-#ifdef CONFIG_ACPI_NUMA
+#if defined(CONFIG_ACPI_NUMA) && !defined(CONFIG_HAVE_ARCH_PARSE_SRAT)
 /*
  * Dummy on 32-bit, for now:
  */