add MonoDevelop.Debugger.Gdb so that it becomes part of the tarball

this commit https://github.com/mono/monodevelop/commit/d60a13af675e68705083ec78861ea7ec0d9a9a7b#diff-11 moved the gdb debugger to the main solution, but the gdb debugger was not added to the tarball.

So when building from the tarball, you would get:
````
[00518] config.status: creating src/addins/MonoDevelop.XmlEditor/Makefile
[00518] config.status: creating src/addins/MonoDevelop.Refactoring/Makefile
[00518] config.status: creating src/addins/MonoDevelop.Debugger/Makefile
[00518] config.status: error: cannot find input file: `src/addins/MonoDevelop.Debugger.Gdb/Makefile.in'
[00518] error: Bad exit status from /var/tmp/rpm-tmp.5MPNEg (%build)
````

My commit adds the gdb debugger to the tarball.
I have tested it on my build server.