sh: Fix up nommu build for out-of-line pgtable changes.

pgtable_cache_init() has been moved out-of-line, so we also need a dummy
definition for it on nommu to fix up the build.
Signed-off-by: Paul Mundt <lethal@linux-sh.org>

arch/sh/mm/nommu.c

@@ -94,3 +94,7 @@ void __init page_table_range_init(unsigned long start, unsigned long end,
 void __set_fixmap(enum fixed_addresses idx, unsigned long phys, pgprot_t prot)
 {
 }
+
+void pgtable_cache_init(void)
+{
+}