Move declaration of ecpg_gettext() to a saner place.

Declaring this in the client-visible header ecpglib.h was a pretty
poor decision.  It's not meant to be application-callable (and if
it was, putting it outside the extern "C" { ... } wrapper means
that C++ clients would fail to call it).  And the declaration would
not even compile for a client, anyway, since it would not have the
macro pg_attribute_format_arg().  Fortunately, it seems that no
clients have tried to include this header with ENABLE_NLS defined,
or we'd have gotten complaints about that.  But we have no business
putting such a restriction on client code.

Move the declaration to ecpglib_extern.h, since in fact nothing
outside src/interfaces/ecpg/ecpglib/ needs to call it.

The practical effect of this is just that clients can now safely
#include ecpglib.h while having ENABLE_NLS defined, but that seems
like enough of a reason to back-patch it.

Discussion: https://postgr.es/m/20590.1573069709@sss.pgh.pa.us