python: Fix build in certain cases.

* lang/python/setup.py.in: Prepend the Python build dir to the list of
include directories so that it takes precedence over any other include
directory.

Fixes the build in case an older 'gpgme.h' is installed and is picked
up by the compiler when compiling the Python module.

• Signed-off-by: Justus Winter <justus@g10code.com>