Add schemaview extension to VisualStudio.gitignore

Files ending in .dbproj.schemaview contain state persistence settings
for the Schema View utility within Visual Studio.

http://stackoverflow.com/questions/3937160/in-the-new-visual-studio-2010-sql-server-project-type-what-is-the-dbproj-schem
http://foocompelsyou.wordpress.com/2012/05/06/visual-studio-2010-database-projects-why-use-them/