Explicitely define LockFile::operator!=

It should be syntethised in terms of `operator==`, but the GCC version
used on aarch64-linux doesn't implement that (see
https://hydra.nixos.org/build/214848896=. So explicitely define it.

Fix https://github.com/NixOS/nix/issues/8159

src/libexpr/flake/lockfile.cc

@@ -234,6 +234,11 @@ bool LockFile::operator ==(const LockFile & other) const
     return toJSON() == other.toJSON();
 }
 
+bool LockFile::operator !=(const LockFile & other) const
+{
+    return !(*this == other);
+}
+
 InputPath parseInputPath(std::string_view s)
 {
     InputPath path;