Correct silly type-o.  Thanks Richard.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@176568 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/include/bitset b/include/bitset
index 06fd729..aa7a596 100644
--- a/include/bitset
+++ b/include/bitset
@@ -253,7 +253,7 @@
     : __first_{__v}
 #elif __SIZE_WIDTH__ == 32
     : __first_{__v, __v >> __bits_per_word}
-#elif
+#else
 #error This constructor has not been ported to this platform
 #endif
 #endif