build: Fix build after recent PipeWire header changes

PipeWire headers were recently cleaned up to reduce the number of
included headers. This leads to a number of functions and types not
being included when needed in wireplumber.

lib/wp/proxy-node.c

@@ -10,6 +10,7 @@
 #include "error.h"
 
 #include <pipewire/pipewire.h>
+#include <spa/pod/builder.h>
 
 struct _WpProxyNode
 {