Missing declaration of _PG_init()

I am writing an extension. The docs describe a _PG_init function that will
be called upon
loading the shared library (https://www.postgresql.org/docs/12/xfunc-c.html)
I include
postgres.h and fmgr.h, but on compilation, _PG_init has not been declared.
Grepping the postgres source, _PG_init appears to be involved in
programming language extensions, and the function is declared in plpgsql.h.
Looking at various contrib modules, I see
explicit declarations of _PG_init(void).

Should _PG_init(void) be declared in someplace included by postgres.h or
fmgr.h?

Jack Orenstein