When building on bionic, -lrt and -lutil only cause a build failure.
Dropping those fixes the build, so it appears that the symbols are defined
in the main library.

This commit moves -lrt and -lutil under a !IS_BIONIC check.
Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>