wvstreams: not available for static only builds

Also propagated to wvdial as it selects wvstreams.

Fixes:
 http://autobuild.buildroot.org/results/0cd/0cd5e0263b42e6845504d97688632e16206bd000/
 http://autobuild.buildroot.org/results/4e0/4e0ab461ea7ed7a288303741eec89156abd49864/



Signed-off-by: Gergely Imreh <imrehg@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>