fix bug that inter-state import of same symbol is possible but causes error.

``` lua
local s1, s2 = ffi.newstate(), ffi.newstate()

s1:copt { cc = "gcc" }
s2:copt { cc = "gcc" }

s1:import({"nanosleep"}):from [[
	#include <sys/types.h>
	#include <sys/time.h>
]]
s2:import({"kevent"}):from [[
	#include <sys/types.h>
	#include <sys/time.h>
	#include <sys/event.h>
]] --[[ causes error attempt to redefine 'timespec' at line 25, 
because previous ffiex cannot check the attempt to import same symbol over 2 state. 
its basically blocked for same state, because of C style include guard.]]
```