Add .gitattributes file

[ulmus-scott]

Found with git grep -i rtsp -- ':!*/external/*'

Checklist

• [x] contribution does not duplicate one of our existing pull requests
• [x] contribution is in a branch rebased against master
• [x] code compiles successfully without errors
• [x] code follows the MythTV Coding Standards
• [x] documentation added/updated/removed where necessary
• [x] commits are logically organised and have good commit messages