802.11: Add ieee80211_supported_rates_vals_ext to be exported

It will be reused by CAPWAP dissector (* Rates Message Element)

Change-Id: I60ce12f382a35cdc2747baf23e2e3c30a305a8bd
Reviewed-on: https://code.wireshark.org/review/5640
Petri-Dish: Michael Mann <mmann78@netscape.net>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Michael Mann <mmann78@netscape.net>