fwupd: Bump dependency to 1.5.6 and drop old conditional code
Debian Stable now ships fwupd 1.5.7, so we can depend on at least that. The highest dependency in our conditional code which was ≤ 1.5.7 was 1.5.6, so set the build dependency to that.
Signed-off-by: Philip Withnall pwithnall@endlessos.org