Fix mapi code generator for out-of-tree build

Fix mapi code generator for out-of-tree build

Use os.path.join() rather than hand-rolling it, so path is correct if
sys.argv[0] returns an absolute path.

(According to the python documentation, it's platform dependent whether
sys.argv[0] is a full pathname or not.  It probably also depends on how
the process was started...)

Signed-off-by: Jon TURNEY <jon.turney@dronecode.org.uk>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>