wireshark: update help text

Since commit 00be67d8 (wireshark: enable GUI options, 2015-12-02) the GUI
is enabled when the required libraries are present. Update the config help
text accordingly.

Cc: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Acked-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>