wireshark: explicitly disable libsmi

wireshark's libsmi detection is not cross compile friendly. We don't have
libsmi in Buildroot yet anyway, so just disable support for it in wireshark.

Fixes
http://autobuild.buildroot.net/results/0daaa3dd74315ac901920e7daf89620ef4c088a0



Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>