libgeom(3): note that stdio.h is required when referencing gctl_dump(3)

gctl_dump(3) is only exposed when stdio.h is #include'd first, per its
addition in r112510. The reasoning noted for the conditional "exposure"
of the function was to "limit #include pollution".

This addresses an issue I found with the documentation when looking at
bug 218809, which in turn addresses a -Wimplicit-function-declaration
compiler warning in tools/regression/geom_gpt/test.c (it uses
gctl_dump(3)).

MFC after: 5 weeks
Sponsored by: Dell EMC Isilon