Fix const default initialization of lock_guard<>

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@272804 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/include/mutex b/include/mutex
index 1d038b5..c047cf9 100644
--- a/include/mutex
+++ b/include/mutex
@@ -617,7 +617,7 @@
 template <>
 class _LIBCPP_TYPE_VIS_ONLY lock_guard<> {
 public:
-    explicit lock_guard() = default;
+    explicit lock_guard() {}
     ~lock_guard() = default;
 
     _LIBCPP_INLINE_VISIBILITY