Fix unused private field warning of FastPthreadMutex (#2839) #1583

Job	Run time
clang-compile-with-bazel	16m 7s
clang-compile-with-bazel-all-options	16m 9s
clang-compile-with-boringssl	16m 59s
clang-compile-with-cmake	3m 12s
clang-compile-with-cmake-all-options	4m 53s
clang-compile-with-make	3m 30s
clang-compile-with-make-all-options	5m 16s
clang-unittest	15m 38s
gcc-compile-with-bazel	16m 36s
gcc-compile-with-bazel-all-options	18m 8s
gcc-compile-with-boringssl	18m 7s
gcc-compile-with-cmake	3m 17s
gcc-compile-with-cmake-all-options	5m 14s
gcc-compile-with-make	2m 40s
gcc-compile-with-make-all-options	5m 0s
	2h 30m 46s

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