setuptools3: pass -j to build step

setuptools3: pass -j to build step

Explicitly call the build command, as by doing this we get to pass -j.

In packages without any C extensions this is mostly a no-op as
bdist_wheel will call it anyway. However, this does make a big
difference to build time for packages with non-trivial C extensions:

  python3-cython:do_compile     204.8s -> 70.9s
  python3-lxml:do_compile       157.5s -> 92.4s

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>