[1955] Added "const" to arguments of setData()

Definition now matches the declaration.

src/lib/dhcp/option.h

@@ -250,8 +250,8 @@ public:
     ///
     /// @param first iterator pointing begining of buffer to copy.
     /// @param last iterator pointing to end of buffer to copy.
-    void setData(OptionBufferConstIter first,
-                 OptionBufferConstIter last);
+    void setData(const OptionBufferConstIter first,
+                 const OptionBufferConstIter last);
 
     /// just to force that every option has virtual dtor
     virtual ~Option();