update gitignore

Do not track *.pyc files (from modules embedded in Buildroot source
tree).

Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>