Fix '_SdkBeforeClean' build error when building xamarin xplat app

SdkBeforeCleanFix.txt

It seems to only be a problem on xplat solutions, as they set up a chain of 'clean'-task dependencies, 
which apparently other solutions don't.

This is caused by a missing `;` in one of the msbuild `.targets` files as described here 
(and is a bug in dotnet core 2.1.3 sdk, and VS 2017 15.8.x):

* https://github.com/onovotny/MSBuildSdkExtras/issues/89 

which led to

* https://github.com/dotnet/sdk/issues/2387

which led to 

* https://github.com/dotnet/sdk/pull/2388/commits/507a2fdeb08a3c73cd1dac235fee86f625bb63a1


The solution is simply to locate the file (`Microsoft.Net.Sdk.targets`), and add the missing semicolon as shown here:

Mine was in the `C:\Program Files\dotnet\sdk\2.1.300\Sdks\Microsoft.NET.Sdk\targets\Microsoft.Net.Sdk.targets`  
around Line 95:

```xml
  <PropertyGroup>
    <CoreCleanDependsOn>
      _SdkBeforeClean; <--- added semicolon here
      $(CoreCleanDependsOn)
    </CoreCleanDependsOn>
  </PropertyGroup>
```

NB: there's a TON of `Microsoft.Net.Sdk.targets` files, so go looking for one(s) that contain `_SdkBeforeClean`.