pgsql: Split out pg_operator.h function declarations to new file pg_ope

Split out pg_operator.h function declarations to new file pg_operator_fn.h.

Commit a2e35b53c39b2a27 added an #include of catalog/objectaddress.h to
pg_operator.h, making it impossible for client-side code to #include
pg_operator.h.  It's not entirely clear whether any client-side code needs
to include pg_operator.h, but it seems prudent to assume that there is some
such code somewhere.  Therefore, split off the function definitions into a
new file pg_operator_fn.h, similarly to what we've done for some other
catalog header files.

Back-patch of part of commit 0dab5ef39b3d9d86.

Branch
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REL9_5_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/2d774aaf1801b0338241cca9409803451ef28a6a

Modified Files
--------------
src/backend/catalog/pg_operator.c    |    1 +
src/backend/commands/operatorcmds.c  |    1 +
src/include/catalog/pg_operator.h    |   17 -----------------
src/include/catalog/pg_operator_fn.h |   32 ++++++++++++++++++++++++++++++++
4 files changed, 34 insertions(+), 17 deletions(-)