Bump external/libbtf from `760a804` to `0a4ef1f` #1525

Job	Run time
build_ubuntu (Debug, tests)	20m 44s
build_windows (Debug, tests)	44m 25s
build_ubuntu (Debug, library)	58s
build_windows (Debug, library)	3m 47s
build_ubuntu (Release, tests)	5m 50s
build_windows (Release, tests)	10m 36s
build_ubuntu (Release, library)	1m 30s
build_windows (Release, library)	5m 11s
	1h 33m 1s

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