* fix chrono cast for llvm

https://stackoverflow.com/a/18284105

tested: clang-3.9, clang-5.0

```
/Source/dlib/dir_nav/dir_nav_kernel_2.h:397:31: error: no viable overloaded '+='
                last_modified += std::chrono::nanoseconds(buffer.st_atim.tv_nsec);
                ~~~~~~~~~~~~~ ^  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/v1/chrono:782:43: note: candidate function not viable: no known conversion from 'duration<[...], ratio<[...], 1000000000>>' to 'const duration<[...], ratio<[...], 1000000>>' for 1st argument
    _LIBCPP_INLINE_VISIBILITY time_point& operator+=(const duration& __d) {__d_ += __d; return *this;}
                                          ^
Source/dlib/dir_nav/dir_nav_kernel_2.cpp:69:33: error: no viable overloaded '+='
            state.last_modified += std::chrono::nanoseconds(buffer.st_atim.tv_nsec);
            ~~~~~~~~~~~~~~~~~~~ ^  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/v1/chrono:782:43: note: candidate function not viable: no known conversion from 'duration<[...], ratio<[...], 1000000000>>' to 'const duration<[...], ratio<[...], 1000000>>' for 1st argument
    _LIBCPP_INLINE_VISIBILITY time_point& operator+=(const duration& __d) {__d_ += __d; return *this;}
```

* fix typo