flatpak-context: Update to latest upstream version
The latest upstream version of flatpak-context.c has a fix for clang 11 that fixes a warning (treated as error in pressure-vessel) about the cast of a pointer to an int.
This MR syncs us to the patch that I sent upstream, that has already being merged https://github.com/flatpak/flatpak/commit/a17224cd08abf1c0f2223d2642a33018f7782fe6
/cc @smcv