Add possibly missing headers for pthread.h

In some environment pthread.h is not defined but its definitions
are used in some headers.
Actually happens using MingW.

Signed-off-by: Frediano Ziglio <fziglio@redhat.com>
Acked-by: Christophe Fergeau <cfergeau@redhat.com>

server/dispatcher.h

@@ -19,6 +19,7 @@
 #ifndef DISPATCHER_H_
 #define DISPATCHER_H_
 
+#include <pthread.h>
 #include <glib-object.h>
 
 #include "red-common.h"

server/image-encoders.h

@@ -20,6 +20,7 @@
 #define IMAGE_ENCODERS_H_
 
 #include <setjmp.h>
+#include <pthread.h>
 #include <common/quic.h>
 #include <common/lz.h>