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

lib/libgeom/libgeom.3

@@ -63,6 +63,10 @@
 .Sh LIBRARY
 .Lb libgeom
 .Sh SYNOPSIS
+.Bd -literal
+/* stdio.h is only required for `gctl_dump` */
+.Ed
+.In stdio.h
 .In libgeom.h
 .Ss "Statistics Functions"
 .Ft void