Correct silly type-o. Thanks Richard.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@176568 91177308-0d34-0410-b5e6-96231b3b80d8

include/bitset

@@ -253,7 +253,7 @@ __bitset<_N_words, _Size>::__bitset(unsigned long long __v) _NOEXCEPT
     : __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