Fix warning about extern with nightly. #2182

Job	Run time
Format	12s
Docs	49s
Examples Rust nightly	3m 33s
Test forcing wrapper generation	6m 38s
Examples Rust beta	3m 25s
Outdated	11s
Examples Rust stable	3m 39s
Examples macOS	3m 0s
Address Sanitizer	13m 45s
Test Rust nightly	9m 43s
Test Rust beta	9m 3s
Test Rust stable	8m 51s
Examples Windows (gnu)	7m 4s
Test macOS	12m 54s
Examples Windows (msvc)	5m 1s
Test Windows (msvc)	14m 25s
Clippy	52s
	1h 43m 5s

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