LinuxCapture: Add mutex around AVC calls #260

Job	Run time
build-linux (gcc-9)	5m 51s
build-linux-Qt6 (gcc-9)	6m 21s
build-macos-brew (clang-9)	3m 29s
build-macos-brew-Qt6 (clang-9)	5m 26s
build-macos-no-brew (clang-9)	5m 59s
build-macos-no-brew-Qt6 (clang-9)	7m 36s
build-windows-mingw (5.15.2)	14m 25s
build-windows-mingw-Qt6 (6.3.0, qtmultimedia qt5compat)	17m 52s
build-windows-vs (5.15.2)	12m 34s
build-windows-vs-Qt6 (6.3.0, qtmultimedia qt5compat)	19m 22s
	1h 38m 55s

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