The linker option -rpath is required to find libs in staging_dir. Now it
is included when building host modules. Without it the import test of
the _ctypes and _uuid modules would fail. The _ctypes module uses
libffi.so.6 from staging, but OpenSUSE LEAP 15 has libffi.so.7.
It will also fail on LEAP 42.x, Fedora28 and 29 and future or old
versions of Ubuntu.

Fix needed in master and 18.06 branches.

Signed-off-by: Jan Kardell <jan.kardell@telliq.com>