Bump third_party/ot-br-posix/repo from `bd15f71` to `952fc6a` #407

Job	Run time
Build on Darwin (clang, python_lib, simulated)	1h 48m 21s
Build on Linux (fake, gcc_release, clang, simulated)	1h 57m 21s
Build on Linux (coverage)	16m 20s
Build on Linux (gcc_debug)	1h 1m 43s
Build on Linux (python_lib)	24m 0s
Build on Linux (python lighting-app)	7m 4s
	5h 34m 49s

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