Use std::shared_mutex in debug_report

Bug: b/143295577
Change-Id: Iee285c4a7ea42fc25b5463f79894d3cca58f17c1
diff --git a/vulkan/libvulkan/debug_report.h b/vulkan/libvulkan/debug_report.h
index e5b1587..416c0bc 100644
--- a/vulkan/libvulkan/debug_report.h
+++ b/vulkan/libvulkan/debug_report.h
@@ -78,8 +78,7 @@
         VkDebugReportCallbackEXT driver_handle;
     };
 
-    // TODO(b/143295577): use std::shared_mutex when available in libc++
-    mutable std::shared_timed_mutex rwmutex_;
+    mutable std::shared_mutex rwmutex_;
     Node head_;
 };