x86: add acpi_numa_slit_init() dummy implementation on 32-bit

allow CONFIG_ACPI_NUMA builds to succeed on 32-bit.
diff --git a/arch/x86/mm/discontig_32.c b/arch/x86/mm/discontig_32.c
index 435c343..d28987a 100644
--- a/arch/x86/mm/discontig_32.c
+++ b/arch/x86/mm/discontig_32.c
@@ -454,3 +454,12 @@
 
 EXPORT_SYMBOL_GPL(memory_add_physaddr_to_nid);
 #endif
+
+#ifdef CONFIG_ACPI_NUMA
+/*
+ * Dummy on 32-bit, for now:
+ */
+void __init acpi_numa_slit_init(struct acpi_table_slit *slit)
+{
+}
+#endif