fix warnings from rb-sys upgrade #843

Job	Run time
test (ubuntu-latest, 2.6, 1.51)	2m 15s
test (ubuntu-latest, 2.6, stable)	4m 21s
test (ubuntu-latest, 2.7, 1.51)	2m 49s
test (ubuntu-latest, 2.7, stable)	3m 37s
test (ubuntu-latest, 3.0, 1.51)	2m 20s
test (ubuntu-latest, 3.0, stable)	3m 35s
test (ubuntu-latest, 3.1, 1.51)	1m 53s
test (ubuntu-latest, 3.1, stable)	4m 50s
test (ubuntu-latest, 3.2, 1.51)	1m 59s
test (ubuntu-latest, 3.2, stable)	3m 31s
test (ubuntu-latest, head, 1.51)	2m 25s
test (ubuntu-latest, head, stable)	2m 45s
test (macos-latest, 2.6, stable)	5m 16s
test (macos-latest, 2.7, stable)	6m 23s
test (macos-latest, 3.0, stable)	4m 59s
test (macos-latest, 3.1, stable)	6m 13s
test (macos-latest, 3.2, stable)	4m 41s
test (macos-latest, head, stable)	4m 2s
test (windows-latest, 2.6, 1.51)	5m 35s
test (windows-latest, 2.6, stable)	14m 28s
test (windows-latest, 2.7, 1.51)	5m 15s
test (windows-latest, 2.7, stable)	10m 21s
test (windows-latest, 3.0, 1.51)	4m 3s
test (windows-latest, 3.0, stable)	9m 25s
test (windows-latest, 3.1, 1.51)	4m 4s
test (windows-latest, 3.1, stable)	12m 7s
test (windows-latest, 3.2, 1.51)	4m 30s
test (windows-latest, 3.2, stable)	12m 8s
test (windows-latest, mswin, stable)	8m 54s
	2h 38m 44s

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