Ignore Reporting Project Upgrade backup files

The conversion process from the old custom rptproj to the MSBuild based projects creates a .bak backup file

VisualStudio.gitignore

@@ -240,6 +240,7 @@ Backup*/
 UpgradeLog*.XML
 UpgradeLog*.htm
 ServiceFabricBackup/
+*.rptproj.bak
 
 # SQL Server files
 *.mdf