Make value_string_ext const-correct.

There is still some const-incorrect usage of them but those can be ironed
out after this change has been made.

Change-Id: Iba0631c804bdab34d7c0232b49967130e3370488
Reviewed-on: https://code.wireshark.org/review/3199
Petri-Dish: Pascal Quantin <pascal.quantin@gmail.com>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Michael Mann <mmann78@netscape.net>