Missing C++11's std::ctype_base::blank in cpp_regex_traits.hpp

`cpp_regex_traits.hpp` introduces its own locale `ctype` masks on top of the standard ones (starting [here](https://github.com/boostorg/xpressive/blob/956d8df7905f042fda53777ae133eb743f40c489/include/boost/xpressive/traits/cpp_regex_traits.hpp#L139)). It relies on first combining all standard flags ([here](https://github.com/boostorg/xpressive/blob/956d8df7905f042fda53777ae133eb743f40c489/include/boost/xpressive/traits/cpp_regex_traits.hpp#L133)) and then finding some unused bits for its own masks.

The block of masks being or'd is missing `std::ctype_base::blank`, which was introduced in C++11.

As a side note, it all happens to work fine in `libc++` because:

```
00000110 11111111 all_ctype_masks
00000001 00000000 ctype_base::blank

10000000 00000000 non_std_ctype_underscore
01000000 00000000 non_std_ctype_blank
00100000 00000000 non_std_ctype_newline
```

i.e. the mask for `blank` isn't conflicting with the `non_std` ones, but that's, of course, sheer luck.