Hide a static inline from FRONTEND code.

For some reason pg_waldump is including tuptable.h, and the recent
addition of a static inline function to it is causing problems on
older buildfarm members that fail to optimize such functions away
completely.  I wonder if this situation doesn't mean that some header
refactoring is called for ... but as a band-aid, wrap the static
function in "#ifndef FRONTEND".

Discussion: https://postgr.es/m/20180824154237.mabsv6fsz5q37bma@alap3.anarazel.de

src/include/executor/tuptable.h

@@ -180,6 +180,8 @@ extern bool slot_getsysattr(TupleTableSlot *slot, int attnum,
 				Datum *value, bool *isnull);
 extern void slot_getmissingattrs(TupleTableSlot *slot, int startAttNum, int lastAttNum);
 
+#ifndef FRONTEND
+
 /*
  * slot_getallattrs
  *		This function forces all the entries of the slot's Datum/isnull
@@ -192,4 +194,6 @@ slot_getallattrs(TupleTableSlot *slot)
 	slot_getsomeattrs(slot, slot->tts_tupleDescriptor->natts);
 }
 
+#endif
+
 #endif							/* TUPTABLE_H */