Build error on ARM

[fgaz]

For some reason I only get this error on ARM while compiling the new evilpixie release

../src/file_save.cpp: In function 'void SaveLayer(const Layer&, const string&, const ProjSettings&)':
../src/file_save.cpp:107:39: error: no match for 'operator!=' (operand types are 'const Box' and 'Box')
  107 |             if (!grid.Empty() && grid != defaultGrid ) {
      |                                  ~~~~ ^~ ~~~~~~~~~~~
      |                                  |       |
      |                                  |       Box
      |                                  const Box

complete log: https://logs.nix.ci/?key=nixos/nixpkgs.205843&attempt_id=998a9684-4e26-4521-834b-37aeb3264487

[bcampbell]

Hmm. How odd. I had thought that operator!= was implicitly provided when you provided an operator==... but the quick digging I've done just now hints that that might not be the case. Except that all the compilers I've tried it on so far do seem to provide operator!= implicitly ;-) In any case, should be an easy fix, but I might not get a chance to do it for a day or so...