Bump third_party/ot-br-posix/repo from `bd15f71` to `af26b01` #590

Job	Run time
Build on Darwin (clang, python_lib, simulated)	1h 10m 17s
Build on Linux (fake, gcc_release, clang, simulated)	2h 4m 42s
Build on Linux (coverage)	14m 1s
Build on Linux (gcc_debug)	1h 8m 3s
Build on Linux (python_lib)	24m 52s
Build on Linux (python lighting-app)	5m 47s
	5h 7m 42s

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