build-sys: link libsystemd-login with libsystemd-label.la

libsystemd-login.la uses cg_create() that currently seems to be a part
of libsystemd-label.la. However, it doesn't link against that library
and it seems that none of the (unconditional) libraries it uses do. In
the end, people end up getting «undefined reference to `cg_create'»
when trying to build e.g. dbus.
logs-show.c: show all messages for a slice