assimp: Don't set `-Werror` #40

Job	Run time
build (win-libs-vc14-x64, Visual Studio 16 2019, Release, windows-2019, -A x64, thirdparty-win64....	29m 6s
build (win-libs-vc14, Visual Studio 16 2019, Release, windows-2019, -A Win32, thirdparty-win32.zip)	35m 22s
build (darwin-libs-a, Unix Makefiles, Release, macos-latest, -DCMAKE_OSX_ARCHITECTURES="arm64;x86...	55m 10s
build (android-libs-arm64, Unix Makefiles, Release, ubuntu-latest, -DCMAKE_TOOLCHAIN_FILE=$ANDROI...	24m 15s
build (android-libs-arm, Unix Makefiles, Release, ubuntu-latest, -DCMAKE_TOOLCHAIN_FILE=$ANDROID_...	23m 26s
build (android-libs-x86_64, Unix Makefiles, Release, ubuntu-latest, -DCMAKE_TOOLCHAIN_FILE=$ANDRO...	23m 10s
build (android-libs-x86, Unix Makefiles, Release, ubuntu-latest, -DCMAKE_TOOLCHAIN_FILE=$ANDROID_...	22m 38s
	3h 33m 7s

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