library: Add buxton.h to installed header files

Note that eventually bt-daemon.h needs to be folded back within
internal Buxton functionality, as buxton.h now provides the public
client API.

Only buxtonctl (cli) and bt-daemon (bt-daemon) should now use
bt-daemon.h, or indeed the internal data types and functionality.

Signed-off-by: Michael Ikey Doherty <michael.i.doherty@intel.com>