compat_threads.c needs string.h for memset.

src/lib/thread/compat_threads.c

@@ -18,6 +18,8 @@
 #include "lib/log/torlog.h"
 #include "lib/log/util_bug.h"
 
+#include <string.h>
+
 /** Allocate and return a new condition variable. */
 tor_cond_t *
 tor_cond_new(void)