pgsql: Avoid including explain.h in explain_format.h and explain_dr.h

Avoid including explain.h in explain_format.h and explain_dr.h

As per a suggestion from Tom Lane, we do this by declaring "struct
ExplainState" here and refer to that rather than "ExplainState".

Also per Tom, CreateExplainSerializeDestReceiver was still defined
in explain.h in addition to explain_dr.h. Remove leftover prototype.

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: http://postgr.es/m/CA+TgmoYtaad3i21V0jqua-fbr+CR0ix6uBvEX8_s6BG96abd=g@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/77cb08be510623421fc727f35980de5107eea735

Modified Files
--------------
src/backend/commands/explain_dr.c     |  1 +
src/backend/tcop/dest.c               |  2 +-
src/include/commands/explain.h        |  2 --
src/include/commands/explain_dr.h     |  6 +++--
src/include/commands/explain_format.h | 42 ++++++++++++++++++++---------------
5 files changed, 30 insertions(+), 23 deletions(-)