Fix a missing include for X11/BSD builds

Closes #1597

Signed-off-by: falkTX <falktx@falktx.com>

source/utils/CarlaPluginUI.cpp

@@ -1,6 +1,6 @@
 /*
  * Carla Plugin UI
- * Copyright (C) 2014-2021 Filipe Coelho <falktx@falktx.com>
+ * Copyright (C) 2014-2022 Filipe Coelho <falktx@falktx.com>
  *
  * This program is free software; you can redistribute it and/or
  * modify it under the terms of the GNU General Public License as
@@ -19,6 +19,7 @@
 #include "CarlaPluginUI.hpp"
 
 #ifdef HAVE_X11
+# include <pthread.h>
 # include <sys/types.h>
 # include <X11/Xatom.h>
 # include <X11/Xlib.h>