gio tests: add codegen to BUILT_SOURCES

We need to have these in BUILT_SOURCES so that 'make' knows to generate them
before attempting to compile other .c files in the same directory (since some
of these files include the header).

Should fix up remaining issues about partial versions of this file being
included under parallel builds.

https://bugzilla.gnome.org/show_bug.cgi?id=725891