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>
diff --git a/arch/sh/mm/nommu.c b/arch/sh/mm/nommu.c
index ac16c05..7694f50 100644
--- a/arch/sh/mm/nommu.c
+++ b/arch/sh/mm/nommu.c
@@ -94,3 +94,7 @@
 void __set_fixmap(enum fixed_addresses idx, unsigned long phys, pgprot_t prot)
 {
 }
+
+void pgtable_cache_init(void)
+{
+}