gtkdoc: fix gtkdoc header script for structs with inline types

gtkdoc: fix gtkdoc header script for structs with inline types

Doxygen adds unhandled xml output for structs that define types inline, for
example struct Foo { enum { FOO, BAR } baz; }. A type definitions precedes the
members. The script wrongly assumed the first sectiondef child of compounddef
would contain all members, but actually this is the case for sectiondefs with
kind=public-attrib (the sectiondef defining the type has kind=public-type).