VS: Restore .sln support for VS Version Selector

Since commit 3b51343ea1 (VS: Emit UTF-8 BOM for generated solution files,
2019-08-19, v3.16.0-rc1~237^2) the `.sln` file does not work with the
VS Version Selector.  Add a newline after the BOM to restore support.

Fixes: #20725
1 file changed