build-sys: do not include id128-constants.h in the dist archive

File src/python-systemd/id128-constants.h is auto generated and its generation
does not require special tools, only sed.  There is no point in bundling it in
the distribution archive, so let's mark it as nodist_ to have it excluded.

Fixes: https://bugs.freedesktop.org/show_bug.cgi?id=80006
Tested: Successfully ran "make dist" after ./configure --without-python.