Sort input filenames to SHA256STAMP

[whitslack]

Make's wildcard function returns matching file paths in an unpredictable order, thereby causing false mismatches in the produced SHA256STAMPs from system to system. This pull request sorts the file paths given to cat to make the stamps deterministic.

This change is crucial if you really want people to be able to build C-Lightning without having Mako installed.

[cdecker]

ACK bcfdcaa

[rustyrussell]

It's weird, I thought about this but couldn't find anywhere it was a problem.

Even so, this is harmless, so applied!

Thanks!

[whitslack]

It's weird, I thought about this but couldn't find anywhere it was a problem.

@rustyrussell: I noticed it while working on the Gentoo ebuilds for C-Lightning. My main system would build 0.9.1 without running generate-wire.py, but a chroot system I have for testing the stable Gentoo environment was always running generate-wire.py. I noticed that the only changes to the generated sources were the SHA256STAMPs, and I had a hunch about the ordering of the dependencies, so I sha256sum'd every permutation of the dependency files and found both the checked-in SHA256STAMP and the one that my build had stamped, so this told me my hunch was right.