Fix test case so that it works on x86-mingw32 platform.

Currently the specs fail with one failure on x86-mingw32:
```
  1) Custom type definitions detects the correct type for size_t
     Failure/Error: lambda do
       expected no Exception, got #<FFI::NotFoundError: Function 'read' not found in [msvcrt.dll]> with backtrace:
         # ./lib/ffi/library.rb:261:in `attach_function'
         # ./spec/ffi/typedef_spec.rb:74:in `block (4 levels) in <top (required)>'
         # ./spec/ffi/typedef_spec.rb:70:in `initialize'
         # ./spec/ffi/typedef_spec.rb:70:in `new'
         # ./spec/ffi/typedef_spec.rb:70:in `block (3 levels) in <top (required)>'
         # ./spec/ffi/typedef_spec.rb:69:in `block (2 levels) in <top (required)>'
     # ./spec/ffi/typedef_spec.rb:69:in `block (2 levels) in <top (required)>'
```

The patch uses ```Platform.windows?```, which means MSVC++ and Mingw32 platforms (but not Cygwin). On these two platforms ```_read()``` can be used, instead.