fix compilation on openbsd #218

Job	Run time
Rustfmt	7s
Check (ubuntu-latest)	10s
Check (windows-latest)	1m 18s
Cross compile (x86_64-unknown-freebsd)	1m 0s
Cross compile (i686-unknown-linux-musl)	40s
Cross compile (aarch64-linux-android)	48s
Cross compile (x86_64-unknown-linux-musl)	53s
Cross compile (armv7-linux-androideabi)	51s
Check (macOS-latest)	21s
Cross compile (x86_64-unknown-netbsd)	46s
	6m 54s

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