python: Fix the MANIFEST file.

This patch adds the Olm header files to the manifest, this results in
the header files being added to source distributions of the python-olm
module.

The headers are required to build the module. Including them in the
source distribution is the easiest way to make sure that builds from the
source distribution will succeed provided that the Olm C library is
already installed.

Signed-off-by: Damir Jelić <poljar@termina.org.uk>

python/MANIFEST.in

-include olm.h
+include include/olm/olm.h
+include include/olm/pk.h
 include Makefile
 include olm_build.py