Const-ify khmer::CountingHash::get_raw_tables meth

This is an UPSTREAM patch

src/ext/oxli/counting.hh

@@ -140,7 +140,7 @@ public:
 
     // Writing to the tables outside of defined methods has undefined behavior!
     // As such, this should only be used to return read-only interfaces
-    Byte ** get_raw_tables()
+    Byte ** get_raw_tables() const
     {
         return _counts;
     }