lint: Add exclusions for libmultiprocess subtree

Without this change linter produces errors about:

- Use of std::filesystem the libmultiprocess example program.
- Use of locale-dependent functions in example program, in the build time code
  generator, and in the runtime library for debug logging.
- Include guards not beginning with BITCOIN_

test/lint/lint_ignore_dirs.py

@@ -2,4 +2,5 @@ SHARED_EXCLUDED_SUBTREES = ["src/leveldb/",
                  "src/crc32c/",
                  "src/secp256k1/",
                  "src/minisketch/",
+                 "src/ipc/libmultiprocess/",
                 ]