It is an interop assembly, it doesn't contain any code. Just declarations, auto-generated from the COM type library. Redistributing is thus fine. The only reason it is installed in the GAC at all is because it is a PIA, a primary interop assembly. You only need a PIA when you write a class library that exposes any of the interfaces in your own public methods. The client of such a library also needs a reference to the interop assembly to be able to use your methods and it must be the exact same one you use. This is rare in general, especially in a BHO.
If you use a Visual Studio version prior to VS2010 then just set the Copy Local property to True for the assembly reference. Rebuild and you'll get a copy of the DLL that you can deploy along with your own executables.
For VS2010 and up, you really want to favor the Embed Interop Type feature. Which embeds the interface declarations in your own assembly, only the ones you actually use. Which helps a great deal to keep the distributable size of your BHO smaller. And avoids the need to deploy that interop assembly. Strongly recommended.
