tdb: define _PUBLIC_ so we can compile tdb.

tdb: define _PUBLIC_ so we can compile tdb.

The Samba tree defines _PUBLIC_ (and _PRIVATE_) for libraries to
control visibility.  The last commit absorbed this from their tdb,
but we need to #define to stub it out since ctdb doesn't use it
(and doesn't need to: we only use tdb internally).

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>