Returns an array of filenames of every file the object's program
actually `#include`d when it was compiled -- nested includes included,
first-seen order, duplicates dropped. The object's own source file is
omitted. Each filename carries a leading slash.
The configured global include file is listed when the compiler opened
it. An `#include` inside a false `#if` branch is not listed: that file
was not opened.
This is the include-side counterpart of `inherit_list()`. Mudlibs that
rebuild only changed objects can walk `include_list(ob)` to see which
headers would force `ob` to recompile.
If no object is supplied, this efun defaults to this_object().