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/test_runner/src/main.rs

@@ -361,6 +361,7 @@ fn lint_std_filesystem() -> LintResult {
             "std::filesystem",
             "--",
             "./src/",
+            ":(exclude)src/ipc/libmultiprocess/",
             ":(exclude)src/util/fs.h",
         ])
         .status()