build: don't uselessly link against dl and rt libraries
dpkg-shlibdeps reports boothd uses none of the symbols exported
by libdl and librt.
dpkg-shlibdeps: warning: dependency on libdl.so.2 could be avoided if "debian/booth/usr/sbin/boothd" were not uselessly linked against it (they use none of its symbols).
dpkg-shlibdeps: warning: dependency on librt.so.1 could be avoided if "debian/booth/usr/sbin/boothd" were not uselessly linked against it (they use none of its symbols).