systemd: disable link time optimization

As reported in https://bugs.busybox.net/show_bug.cgi?id=7286

,
systemd fails to build with BR2_OPTIMIZE_2=y. This mode activates the GCC
options -flto and -ffat-lto-objects, which do not mix well with the GCC
tricks used in src/compat-libs/linkwarning.h to build the compatibility
libraries.

As a temporary workaround, this patch disables link time optimization for
systemd.

Signed-off-by: Eric Le Bihan <eric.le.bihan.dev@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>