Fix: Unity requires static libs for iOS.

[atteneder]

Changed the library type from MODULE to STATIC for dracodec_unity IOS only. For other platforms it stays MODULE.

Tested with CMake 3.14.5 (has built-in iOS toolchain)

cmake .. -G Xcode -DCMAKE_SYSTEM_NAME=iOS -DCMAKE_OSX_ARCHITECTURES=armv7\;armv7s\;arm64 -DCMAKE_OSX_DEPLOYMENT_TARGET=10.0 -DCMAKE_XCODE_ATTRIBUTE_ONLY_ACTIVE_ARCH=NO -DBUILD_UNITY_PLUGIN=ON

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