ecpg: Move function prototype into header file

Peter Eisentraut <peter_e@gmx.net>

Commit: 9feeef92fb9b73371e3f299ba444f86b4ef7e26f
Author: Peter Eisentraut <peter_e@gmx.net>
Date: 2013-07-23T01:02:56Z
Releases: 9.4.0
ecpg: Move function prototype into header file

PGTYPEStimestamp_defmt_scan() was declared twice inside different .c
files, with slightly different prototypes.  Move it into a header file
and correct the prototype.

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