s3: lib/ctdbd_conn: include .h, not .c

s3: lib/ctdbd_conn: include .h, not .c

Probably a copy/paste mistake. Detected by a failing autobuild on
sn-devel and a local make test:

Build failed: default/examples/libsmbclient/testbrowse2: Symbol
tevent_req_is_unix_error linked in multiple libraries
['samba-cluster-support', 'tevent-util']
UNEXPECTED(failure): wafsamba.duplicate_symbols.duplicate_symbols(none)

Wonder why this didn't fail before in autobuild.

Signed-off-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Alexander Bokovoy <ab@samba.org>
Autobuild-User(master): Ralph Böhme <slow@samba.org>
Autobuild-Date(master): Wed Jul  4 01:19:50 CEST 2018 on sn-devel-144