Link gdbm.so to libbuxton-shared

As counter intuitive as this may seem, gdbm currently needs access
to libbuxton specific functionality, namely buxton_get_name to split
a key's name from the key:group identifier.

In the near future the present make system will be modified to remove
cross-linking between existing components, and shared accessors such
as buxton_get_name will be split into a static library.

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