[Linux] Only export extension init symbol #197

Job	Run time
📊 Static Checks (clang-format, black format, file format)	34s
🔧 Build (android, x86_64, android_arch=x86_64, ubuntu-20.04, android-x86_64)	3m 47s
🔧 Build (android, arm64, android_arch=arm64v8, ubuntu-20.04, android-arm64)	3m 38s
🔧 Build (ios, x86_64, ios_arch=x86_64, ios_simulator=true, macos-11, ios-x86_64-simulator)	10m 51s
🔧 Build (ios, arm64, ios_arch=arm64, macos-11, ios-arm64)	10m 31s
🔧 Build (linux, x86_32, i686, bits=32, ubuntu-20.04, linux-x86_32)	6m 36s
🔧 Build (linux, x86_64, x86_64, bits=64, ubuntu-20.04, linux-x86_64)	6m 15s
🔧 Build (macos, universal, macos_arch=universal bits=64, macos-11, macos-universal)	18m 22s
🔧 Build (windows, x86_32, bits=32, use_mingw=yes, ubuntu-20.04, amd64_x86, win-x86_32)	6m 52s
🔧 Build (windows, x86_64, bits=64, use_mingw=yes, ubuntu-20.04, amd64, win-x86_64)	6m 51s
📦 Package	2m 54s
	1h 17m 11s

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