multiple includes in _stub.c files

[Alastair Reid]

The _stub.c files generated when using the ffi currently contain one #include 
for every foreign import in the .hs file.  For example, 
libraries/Win32/System/Win32/Window_stub.c starts off like this:

#define IN_STG_CODE 0
#include "Stg.h"
#include "HsBase.h"
#include "HsLang.h"
#include "Window_stub_ffi.h"
#include "Window_stub_ffi.h"
#include "Window_stub_ffi.h"
#include "Window_stub_ffi.h"
#include "Window_stub_ffi.h"
#include "Window_stub_ffi.h"
#include "Window_stub_ffi.h"
#include "Window_stub_ffi.h"
#include "Window_stub_ffi.h"
#include "Window_stub_ffi.h"
#include "Window_stub_ffi.h"
#include "Window_stub_ffi.h"
#include "Window_stub_ffi.h"
#include "Window_stub_ffi.h"
#include "Window_stub_ffi.h"
#include "Window_stub_ffi.h"
#include "Window_stub_ffi.h"
#include "Window_stub_ffi.h"
#include "Window_stub_ffi.h"
#include "Window_stub_ffi.h"
#include "Window_stub_ffi.h"
#include "Window_stub_ffi.h"
#include "Window_stub_ffi.h"
#include "Window_stub_ffi.h"
#include "Window_stub_ffi.h"
#include "Window_stub_ffi.h"
#include "Window_stub_ffi.h"
#include "Window_stub_ffi.h"
#include "Window_stub_ffi.h"
#include "Window_stub_ffi.h"
#include "Window_stub_ffi.h"
#include "Window_stub_ffi.h"
#include "Window_stub_ffi.h"
#include "Window_stub_ffi.h"
#include "Window_stub_ffi.h"
#include "Window_stub_ffi.h"
#include "Window_stub_ffi.h"
#include "Window_stub_ffi.h"
#include "Window_stub_ffi.h"
#include "Window_stub_ffi.h"
#include "Window_stub_ffi.h"
#include "Window_stub_ffi.h"
#include "Window_stub_ffi.h"
#include "Window_stub_ffi.h"
#include "Window_stub_ffi.h"
#include "Window_stub_ffi.h"
#include "Window_stub_ffi.h"
#include "Window_stub_ffi.h"
#include "Window_stub_ffi.h"
#include "Window_stub_ffi.h"
#include "Window_stub_ffi.h"
#include "Window_stub_ffi.h"
#include "Window_stub_ffi.h"
#include "Window_stub_ffi.h"
#include "Window_stub_ffi.h"
#include "Window_stub_ffi.h"
#include "Window_stub_ffi.h"
#include "Window_stub_ffi.h"
#include "Window_stub_ffi.h"
#include "Window_stub_ffi.h"
#include "Window_stub_ffi.h"

Suppressing duplicates would be nice.

[At the same time, I'm going to tweak greencard so that the header files it 
generates have multi-include protection.]

--
Alastair