libnl: do not allow to build tools on static lib configs

Building the libnl tools requires the <dlfcn.h> header, which is not
compatible with static only builds. Therefore, this commit makes the
libnl tools option depend on !BR2_PREFER_STATIC_LIB.

Fixes:

   http://autobuild.buildroot.org/results/573/57340b9bd2db3a523836e02e01cba6f7c17c8ca7/build-end.log



[Peter: fix tools comment dependency]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Acked-by: Samuel Martin <s.martin49@gmail.com>