link error missing the "_alloca" on win7 64bit systerm build with 32bit msvc compiler

Issue #18 resolved
created an issue

I try to build the cffi-0.3 with 32bit msvc compiler on win7 64bit system, then I get an link error about missing the _alloca function. I add the header file "malloc.h" into _cffi_backend.c then the compilation is fine.

ifdef MS_WIN32

include <windows.h>

include "misc_win32.h"

include <malloc.h> // add the header file will be ok