Python3: Fix host build on OpenSUSE

Python3: Fix host build on OpenSUSE

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>
(cherry picked from commit 6ade5a1e3adb189d062b707e2b8f731c9faab844)