package/nut: disable libltdl (and nut-scanner) for static-only

nut-scanner needs libltdl, a wrapper around dlopen/dlsym. This does not
really make sense on a static-only build.

Disable use of libltdl in static-only builds; this disble nut-scanner.

Fixes:
    http://autobuild.buildroot.org/results/d8e/d8e769709681b1965f36eb5c27f71f3a1f5adcf1/
    http://autobuild.buildroot.org/results/6e4/6e403642069093389acbeeccd7030b76d4ab68be/
    http://autobuild.buildroot.org/results/fa4/fa4c647ce58b91b863d8de5bf9d44b2060a1cf1c/


    and so many others...

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>