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

Source/cmGlobalVisualStudioGenerator.cxx

@@ -111,6 +111,7 @@ void cmGlobalVisualStudioGenerator::WriteSLNHeader(std::ostream& fout)
 {
   char utf8bom[] = { char(0xEF), char(0xBB), char(0xBF) };
   fout.write(utf8bom, 3);
+  fout << '\n';
 
   switch (this->Version) {
     case cmGlobalVisualStudioGenerator::VS9: