Add the xml namespace to work with MSBuild v14

[jbaehr]

This fixes #1823.

Tests have been done with with MSBuild-14, MSBuild-15.8 and NetSdk-2.2 (aka MSBuild-15.9).
MSBuild-15 works without this patch, but MSBuild-14 seems to require the namespace, otherwise you'll encounter the following error:

C:\...\GitVersionTask.props(2,1): error MSB4041: The default XML namespace of the project must be the MSBuild XML namespace. If the project is authored in the MSBuild 2003 format, please add xmlns="http://schemas.microsoft.com/developer/msbuild/2003" to the <Project> element. If the project has been authored in the old 1.0 or 1.2 format, please convert it to MSBuild 2003 format. [C:\...\MyProject.csproj]

[arturcic]

@jbaehr, thanks for taking the time to implement it again. I'll merge it when the build passes