python3: fix termios build for xtensa

Forward port the patch from c821210b (python: fix termios build for
xtensa) to python3.

Fixes:
http://autobuild.buildroot.net/results/e7f/e7f71fbcb0796f315052947382fa39d85c5dc8fe/



Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>