Fix compile errors when not using UNITY_STANDALONE or ANDROID platforms

[Wully616]

Fixes #5

Fixes the following compliation errors when unity is set to a platform which is not Standalone or Android.

Library\PackageCache\com.htc.upm.vive.openxr@df2fc918e700\Runtime\CompositionLayer\Scripts\CompositionLayerPassthroughAPI.cs(390,22): error CS0161: 'CompositionLayerPassthroughAPI.DestroyPassthrough(int)': not all code paths return a value

Library\PackageCache\com.htc.upm.vive.openxr@df2fc918e700\Runtime\CompositionLayer\Scripts\CompositionLayerPassthroughAPI.cs(442,22): error CS0161: 'CompositionLayerPassthroughAPI.SetPassthroughAlpha(int, float, bool)': not all code paths return a value

Library\PackageCache\com.htc.upm.vive.openxr@df2fc918e700\Runtime\CompositionLayer\Scripts\CompositionLayerPassthroughAPI.cs(498,22): error CS0161: 'CompositionLayerPassthroughAPI.SetProjectedPassthroughMesh(int, Vector3[], int[], bool)': not all code paths return a value

Library\PackageCache\com.htc.upm.vive.openxr@df2fc918e700\Runtime\CompositionLayer\Scripts\CompositionLayerPassthroughAPI.cs(578,22): error CS0161: 'CompositionLayerPassthroughAPI.SetProjectedPassthroughMeshTransform(int, ProjectedPassthroughSpaceType, Vector3, Quaternion, Vector3, bool, bool)': not all code paths return a value

Library\PackageCache\com.htc.upm.vive.openxr@df2fc918e700\Runtime\CompositionLayer\Scripts\CompositionLayerPassthroughAPI.cs(644,22): error CS0161: 'CompositionLayerPassthroughAPI.SetPassthroughLayerType(int, LayerType, uint)': not all code paths return a value

Library\PackageCache\com.htc.upm.vive.openxr@df2fc918e700\Runtime\CompositionLayer\Scripts\CompositionLayerPassthroughAPI.cs(679,22): error CS0161: 'CompositionLayerPassthroughAPI.SetProjectedPassthroughSpaceType(int, ProjectedPassthroughSpaceType)': not all code paths return a value

Library\PackageCache\com.htc.upm.vive.openxr@df2fc918e700\Runtime\CompositionLayer\Scripts\CompositionLayerPassthroughAPI.cs(726,22): error CS0161: 'CompositionLayerPassthroughAPI.SetProjectedPassthroughMeshPosition(int, Vector3, bool, bool)': not all code paths return a value

Library\PackageCache\com.htc.upm.vive.openxr@df2fc918e700\Runtime\CompositionLayer\Scripts\CompositionLayerPassthroughAPI.cs(788,22): error CS0161: 'CompositionLayerPassthroughAPI.SetProjectedPassthroughMeshOrientation(int, Quaternion, bool, bool)': not all code paths return a value

Library\PackageCache\com.htc.upm.vive.openxr@df2fc918e700\Runtime\CompositionLayer\Scripts\CompositionLayerPassthroughAPI.cs(843,22): error CS0161: 'CompositionLayerPassthroughAPI.SetProjectedPassthroughScale(int, Vector3)': not all code paths return a value