vm_dump.c: avoid `USE_MN_THREADS` is not defined warning #652

Job	Run time
make (test-bundler-parallel, 50)	26m 8s
make (check, ubuntu-24.04)	8m 19s
make (check, i686)	12m 15s
make (check, --enable-shared --enable-load-relative)	7m 59s
make (check, --disable-yjit)	6m 35s
make (test-bundled-gems)	16m 45s
make (check, cppflags=-DVM_CHECK_MODE)	7m 57s
make (check, ubuntu-24.04-arm)	6m 52s
Ubuntu result	3s
	1h 32m 53s

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