pgsql: Use PqMsg_* macros in fe-protocol3.c.

Use PqMsg_* macros in fe-protocol3.c.

Oversight in commit f4b54e1ed9.

Reviewed-by: Jacob Champion <jacob.champion@enterprisedb.com>
Reviewed-by: Fabrízio de Royes Mello <fabriziomello@gmail.com>
Discussion: https://postgr.es/m/aKx5vEbbP03JNgtp%40nathan

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/3ef2b863a376af35e8d5f2cd1bb4311ec7a9f299

Modified Files
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src/include/libpq/protocol.h        |  1 +
src/interfaces/libpq/fe-protocol3.c | 12 ++++++------
2 files changed, 7 insertions(+), 6 deletions(-)