Bump external/libbtf from `2de28d7` to `2f68368` #1463

Job	Run time
build_ubuntu (Debug, tests)	26m 36s
build_windows (Debug, tests)	35m 36s
build_ubuntu (Debug, library)	1m 19s
build_windows (Debug, library)	4m 28s
build_ubuntu (Release, tests)	5m 44s
build_windows (Release, tests)	9m 39s
build_ubuntu (Release, library)	1m 21s
build_windows (Release, library)	5m 48s
	1h 30m 31s

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