Bump external/libbtf from `2de28d7` to `21f05ef` #160

Job	Run time
build_ubuntu (Debug, tests)	32m 52s
build_ubuntu (Debug, library)	1m 9s
build_windows (Debug, tests)	43m 57s
build_ubuntu (Release, tests)	6m 14s
build_windows (Debug, library)	4m 23s
build_ubuntu (Release, library)	1m 37s
build_windows (Release, tests)	11m 25s
build_windows (Release, library)	5m 36s
	1h 47m 13s

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