libselinux: needs shared library support

Even though technically libselinux could make its <dlfcn.h> include
optional, the build system isn't really suited to build and install
only the static variant of libselinux, so let's make libselinux and
its reverse dependency not available in pure-static environments.

Fixes:

  http://autobuild.buildroot.org/results/90d/90dc73980a45b9b0441be3d493b22e3afea3cd6e/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>