Fix compile error when building on SDK 11.1
Fix compile error when building on SDK 11.1

SDK 11.1 and SDK 12 have different declaration for MTLDevice::supportsBCTextureCompression. 
On SDK 11.1 it is marked as API_AVAILABLE for ios and on SDK 12 is marked as API_UNAVAILABLE ios. 
Guard the API_UNAVAILABLE(ios) declaration to only be included when compiling for SDK 12.0 or above.

The compile error message before the fix:

renderdoc/driver/metal/metal_device_bridge.mm:158:64: error: method cannot be unavailable on iOS when the protocol method it implements is available [-Werror,-Wavailability]
69
- (BOOL)supportsBCTextureCompression API_AVAILABLE(macos(11.0))API_UNAVAILABLE(ios)

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