Bump external/libbtf from `2f68368` to `81e0a6f` #1478

Job	Run time
build_ubuntu (Debug, tests)	19m 46s
build_ubuntu (Debug, library)	1m 7s
build_windows (Debug, tests)	39m 33s
build_ubuntu (Release, tests)	5m 56s
build_windows (Debug, library)	4m 32s
build_ubuntu (Release, library)	1m 16s
build_windows (Release, tests)	11m 16s
build_windows (Release, library)	5m 5s
	1h 28m 31s

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