include virterror_internal.h in threads.h

required by VIR_ONCE_GLOBAL_INIT using virSetError.

src/util/virthread.h

@@ -23,6 +23,7 @@
 # define __THREADS_H_
 
 # include "internal.h"
+# include "virerror.h"
 
 typedef struct virMutex virMutex;
 typedef virMutex *virMutexPtr;