pgsql: Move AllowCascadeReplication() define from xlog.h to replication

Move AllowCascadeReplication() define from xlog.h to replication include
file.

Per suggestion from Alvaro.

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/85e6e1662b2c475a408afc1ea2ddade7974eddbb

Modified Files
--------------
src/include/access/xlog.h | 3 ---
src/include/replication/walreceiver.h | 4 ++++
src/include/replication/walsender.h | 1 +
3 files changed, 5 insertions(+), 3 deletions(-)