s390/mm: provide emtpy check_pgt_cache() function

All architectures need to provide a check_pgt_cache() function. The s390 one
got lost somewhere.
So reintroduce it to prevent future compile errors e.g. if Thomas Gleixner's
idle loop rework patches get merged.
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/include/asm/pgtable.h

@@ -1532,7 +1532,8 @@ extern int s390_enable_sie(void);
 /*
  * No page table caches to initialise
  */
-#define pgtable_cache_init()	do { } while (0)
+static inline void pgtable_cache_init(void) { }
+static inline void check_pgt_cache(void) { }
 
 #include <asm-generic/pgtable.h>