nghttp2: Disable python bindings

python bindings need cython to build, moreover they have been removed
from future releases upstream nghttp2 [1]

This ensures consistent builds as it does not need cython which was
being used from build host if it is installed. With some distros e.g.
archlinux it caused conflicts with python from python-native

[1] https://github.com/nghttp2/nghttp2/commit/b4cb3b0090b704f2461766c455568b311013272f



Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>