How to add built-in func?

I only add below:

Datum fake_dinstance2(PG_FUNCTION_ARGS)
{
PG_RETURN_INT16(0);
}
in src/backend/utils/adt/int8.c, and the I run “make install”,
But I can’t find the fake_distance2 in src/backend/utils/fmgrtab.c which is
generated by src/backend/utils/Gen_fmgrtab.pl. What else do I need to add?