vm_dump.c: avoid `USE_MN_THREADS` is not defined warning #556

Job	Run time
cargo clippy	52s
make (yjit-bindgen, To fix: use patch in logs, --with-gcc=clang-14 --enable-yjit=dev, /usr/lib/ll...	4m 33s
make (check, RUSTC='rustc +1.58.0', 1.58.0)	7m 49s
make (check, --enable-yjit=dev, --yjit-call-threshold=1 --yjit-verify-ctx --yjit-code-gc, -T --yj...	15m 6s
make (yjit-bench, --enable-yjit=dev, --yjit-stats, true)	5m 33s
make (check, --enable-yjit=dev)	11m 1s
make (test-bundled-gems, --enable-yjit=dev)	18m 54s
cargo test	1m 3s
YJIT Ubuntu result	2s
	1h 4m 53s

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