Add missing #include.

--- aseqview-0.2.8/portlib.c	2007-04-03 14:02:03.000000000 +0100
+++ aseqview-0.2.8/portlib.c	2025-07-05 15:14:06.205282418 +0100
@@ -20,6 +20,7 @@
 #include <unistd.h>
 #include <stdlib.h>
 #include <string.h>
+#include <pthread.h>
 #include "portlib.h"