wireshark: disable build of user guide

--disable-guides has been added since version 2.5.0:
https://github.com/wireshark/wireshark/commit/aa04d2355effb1cf611831b0a260cf1aff199b9a

Fixes:
 - http://autobuild.buildroot.org/results/5ae6d46573942713687f4e659b3eaf0b768ebc71



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>