Bump third_party/ot-br-posix/repo from `bd15f71` to `5c66507` #410

Job	Run time
Build on Darwin (clang, python_lib, simulated)	1h 42m 54s
Build on Linux (fake, gcc_release, clang, simulated)	1h 58m 2s
Build on Linux (gcc_debug)	1h 0m 0s
Build on Linux (python_lib)	23m 50s
Build on Linux (coverage)	14m 6s
Build on Linux (python lighting-app)	6m 50s
	5h 25m 42s

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