python: Read gpg-error.h using the pre-processor

* lang/python/setup.py.in: Read gpg-error.h using the pre-processor.

The libgpg-error may be installed in multilib configuration in which
there is a wrapper header at /usr/include that includes the actual
header at /usr/include/*. This causes invalid errors.i generation.

Let the pre-processor extract the header content instead reading it
explicitly.

• Signed-off-by: Alon Bar-Lev <alon.barlev@gmail.com>