bt2: by-pass distutils changed file check

Changes made to a SWIG interface file (.i) that is included in
native_bt.i would not trigger rebuilding of the extension on `make`
as the main source file (native_bt.i) has not changed.

The Makefile is already tracking when files changed and need to be
recompiled so we can simply bypass the setup.py checks by using the
--force flag.

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>