Remote Process Dialog: Update buttons if sysroot changes

Make sure 'Attach to Selected Process' is enabled again e.g. if first attaching
fails because of a missing sysroot, and the user then sets the sysroot.

Change-Id: I1aa949bfe0aa41186a76e7f683dd29070a6b63d4
Reviewed-by: hjk <qthjk@ovi.com>