Bump external/libbtf from `0a4ef1f` to `3c2012b` #1534

Job	Run time
build_ubuntu (Debug, tests)	20m 38s
build_windows (Debug, tests)	44m 56s
build_ubuntu (Debug, library)	1m 2s
build_windows (Debug, library)	4m 42s
build_ubuntu (Release, tests)	6m 8s
build_windows (Release, tests)	10m 38s
build_ubuntu (Release, library)	1m 6s
build_windows (Release, library)	5m 29s
	1h 34m 39s

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