Bump external/libbtf from `21f05ef` to `3c0cbd6` #1499

Job	Run time
build_windows (Debug, tests)	42m 22s
build_ubuntu (Debug, tests)	21m 43s
build_windows (Debug, library)	4m 37s
build_ubuntu (Debug, library)	1m 3s
build_windows (Release, tests)	10m 12s
build_windows (Release, library)	5m 0s
build_ubuntu (Release, tests)	6m 26s
build_ubuntu (Release, library)	1m 10s
	1h 32m 33s

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