Add gobject example source code to path for mkhtml test

Fixes #150

I'm not really sure why this is required, but I suspect it has to do with changes in how GCC handles path traversal due to recent (ish) security issues, as the code here is evaluated to ../xml/../../examples/gobject.c which probably looks unhealthy to automatically include.

Signed-off-by: Neil Hanlon neil@shrug.pw