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