fixes the bitrig d_namlen compile error

[dhuseby]

@alexcrichton this fixes the current build breakage for the bitrig build:

./src/libstd/sys/unix/fs.rs:221:37: 221:56 error: attempted access of field `d_namlen` on type `libc::unix::bsd::openbsdlike::bitrig::dirent`, but no field with that name was found
../src/libstd/sys/unix/fs.rs:221                                     self.entry.d_namlen as usize)
                                                                     ^~~~~~~~~~~~~~~~~~~
../src/libstd/sys/unix/fs.rs:221:48: 221:56 help: did you mean `d_namelen`?
../src/libstd/sys/unix/fs.rs:221                                     self.entry.d_namlen as usize)
                                                                                ^~~~~~~~