Consistently label __bit_array as a struct, not a class.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@162108 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/include/vector b/include/vector
index 04a23ab..07859c9 100644
--- a/include/vector
+++ b/include/vector
@@ -2318,7 +2318,7 @@
     friend class __bit_const_reference<vector>;
     friend class __bit_iterator<vector, false>;
     friend class __bit_iterator<vector, true>;
-    friend class __bit_array<vector>;
+    friend struct __bit_array<vector>;
     friend struct _LIBCPP_VISIBLE hash<vector>;
 };