etc: Add helgrind suppression for lock_held_by_current_thread

Author: brson

src/etc/x86.supp

@@ -380,3 +380,10 @@
    ...
    fun:uv_loop_delete
 }
+
+{
+   probably-threadsafe-access-outside-of-lock
+   Helgrind:Race
+   fun:_ZN15lock_and_signal27lock_held_by_current_threadEv
+   ...
+}