python bindings: activate thread-safety code generation

The Parameter Framework may spawn a thread listening to request for command
execution. In this case, whenever a command produces a log, a callback function
provided by the client is called. When the client is a python script using the
python bindings, this causes a python function to be called in a different
context than the main thread. This can only work safely if a specific lock is
held before hand; see

    https://docs.python.org/2/c-api/init.html#non-python-created-threads



Fortunately, SWIG can generate the corresponding code automatically using

    %module(threads=1, ...) mymodule

Signed-off-by: David Wagner <david.wagner@intel.com>