Bump external/libbtf from `2de28d7` to `3c0cbd6` #161

Job	Run time
build_ubuntu (Debug, tests)	33m 7s
build_windows (Debug, tests)	44m 41s
build_ubuntu (Debug, library)	1m 8s
build_windows (Debug, library)	5m 18s
build_ubuntu (Release, tests)	6m 22s
build_windows (Release, tests)	11m 19s
build_ubuntu (Release, library)	1m 41s
build_windows (Release, library)	4m 38s
	1h 48m 14s

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