flatpak: Disable -Werror for gexiv2
AX_COMPILER_FLAGS promotes all warnings to errors for non-release builds, which is a bit too much because it also includes -Wno-deprecated-declarations. It's good to be notified about the use of deprecated API, but there's no reason to fail the nightly build for that. Especially, since not every deprecation is born equal - some may take longer than others to fix.