Add libGL to the Visual Studio solution.

Bug 18962347

Change-Id: Ife807755d9f30aaf8b7b5591254fdeead25b3fbc
Reviewed-on: https://swiftshader-review.googlesource.com/1725
Reviewed-by: Nicolas Capens <capn@google.com>
Tested-by: Nicolas Capens <capn@google.com>