Fix harmless compiler warnings.

FossilOrigin-Name: e54dded2012f0ab486ee138e9bd57c528af33980
diff --git a/src/sqliteInt.h b/src/sqliteInt.h
index b802d7a..092a143 100644
--- a/src/sqliteInt.h
+++ b/src/sqliteInt.h
@@ -3652,8 +3652,10 @@
 /*
 ** Threading interface
 */
+#if SQLITE_MAX_WORKER_THREADS>0
 int sqlite3ThreadCreate(SQLiteThread**,void*(*)(void*),void*);
 int sqlite3ThreadJoin(SQLiteThread*, void**);
+#endif
 
 /*
 ** Win32 interface