LinuxCapture: Add mutex around AVC calls #258

Job	Run time
build-macos-no-brew (clang-9)	57s
build-linux (gcc-9)	5m 22s
build-linux-Qt6 (gcc-9)	6m 32s
build-macos-brew (clang-9)	1m 52s
build-macos-no-brew-Qt6 (clang-9)	1m 7s
build-windows-mingw (5.15.2)	15m 8s
build-windows-mingw-Qt6 (6.3.0, qtmultimedia qt5compat)	17m 13s
build-windows-vs (5.15.2)	11m 56s
build-windows-vs-Qt6 (6.3.0, qtmultimedia qt5compat)	18m 47s
build-macos-brew-Qt6 (clang-9)	2m 28s
	1h 21m 22s

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