lang/mono*: add compat symlink for devel/msbuild < 16

/usr/local/lib/mono/msbuild/15.0/bin/Microsoft.CSharp.CurrentVersion.targets(331,5): error MSB4019: The imported project "/usr/local/lib/mono/msbuild/15.0/bin/Roslyn/Microsoft.CSharp.Core.targets" was not found. Confirm that the path in the <Import> declaration is correct, and that the file exists on disk.

PR: 256342
Suggested by: Marcin Cieślak
Regressed by: https://github.com/mono/mono/commit/1717687e16dd