Add the #includes from the rts package to the stub .c file.

Prior to rev. 1.42, we used to add all the #includes from all enabled
packages, together with any -#include options from the command-line.
But since this is auto-generated code and we know exactly which
#includes are required, this was overkill.

In rev. 1.42, all #includes except RtsAPI.h were removed from the stub
.c file.  This was incorrect, because the stub file refers to some
entities defined in other RTS header files (the StgStablePtr type, and
deRefStablePtr() for example).

This change adds the header files from the rts package to the stub .c
file, fixing some recent test breakages.