Suppress unused variable warning in helper_thread.c #881

Job	Run time
build (linux-gcc)	13m 9s
build (linux-clang)	11m 58s
build (macos)	11m 16s
build (linux-i686-gcc)	13m 1s
build (android)	36s
build (windows-cygwin-64)	15m 7s
build (windows-cygwin-32)	15m 25s
build (windows-msys2-64)	13m 32s
build (android-recovery, ubuntu-22.04, aarch64-linux-android32)	39s
	1h 34m 43s

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