Fix portability problem induced by commit a6f6b7819.

pg_xlogdump includes bufmgr.h.  With a compiler that emits code for
static inline functions even when they're unreferenced, that leads
to unresolved external references in the new static-inline version
of BufferGetPage().  So hide it with #ifndef FRONTEND, as we've done
for similar issues elsewhere.  Per buildfarm member pademelon.

src/backend/storage/buffer/bufmgr.c

@@ -4299,7 +4299,7 @@ IssuePendingWritebacks(WritebackContext *context)
  * For best performance, keep the tests that are fastest and/or most likely to
  * exclude a page from old snapshot testing near the front.
  */
-extern void
+void
 TestForOldSnapshot(Snapshot snapshot, Relation relation, Page page)
 {
 	Assert(relation != NULL);