remove <sys/timeb.h> for configure check

Because following warning is generated in FreeBSD 9.0+:

```log
  CXX      dat.lo
In file included from dat.cpp:17:
In file included from ./groonga_in.h:54:
/usr/include/sys/timeb.h:42:2: warning: "this file includes <sys/timeb.h> which
      is deprecated" [-W#warnings]
#warning "this file includes <sys/timeb.h> which is deprecated"
 ^
1 warning generated.
```

`ftime(3)` is depreacted POSIX API.
Since FreeBSD 9.0, compiler complains \<sys/timeb.h\> is deprecated:
"this file includes \<sys/timeb.h\> which is deprecated"

* In OpenBSD no longer exists this header.
    - removed libcompat at 2013/12/08
* NetBSD's header comment says deprecated, but no warnings during compilation.

It is only used in WIN32 environment.(Windows does not have `gettimeofday(2)`)