Work around broken FreeBSD headers

It seems that including just pthread.h does not define
clockid_t on these systems.

https://bugzilla.gnome.org/show_bug.cgi?id=722526

glib/deprecated/gthread.h

@@ -120,6 +120,7 @@ void     g_thread_foreach      (GFunc             thread_func,
                                 gpointer          user_data);
 
 #ifndef G_OS_WIN32
+#include <time.h>
 #include <pthread.h>
 #endif