Include <arpa/nameser.h> before <resolv.h>.

diff --git a/scheduler/pthread.c b/scheduler/pthread.c
index 1191b35..ab4a3ef 100644
--- a/scheduler/pthread.c
+++ b/scheduler/pthread.c
@@ -48,6 +48,9 @@
 #ifdef HAVE_NETINET_IN_H
 # include <netinet/in.h>
 #endif
+#ifdef HAVE_ARPA_NAMESER_H
+# include <arpa/nameser.h>
+#endif
 #ifdef HAVE_RESOLV_H
 # include <resolv.h>
 #endif