build: ignore the changes in submodules

When we commit the changes in osv.git, we usually make a mistake to
also commit the changes in the submodules. To avoid this, this patch
is trying to add "ignore = all" to .gitmodules file.

Signed-off-by: Zhi Yong Wu <zwu.kernel@gmail.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>