fix(audio): include <spa/pod/iter.h> for spa_pod_find_prop()

audio_object.cpp calls spa_pod_find_prop() but only includes
<spa/pod/parser.h>, so the declaration is missing and newer
pipewire headers no longer pull iter.h in transitively.

Building with GCC 16.2.0 and pipewire 1.4.x fails with:

    clients/audio/audio_object.cpp: In function 'const spa_pod_prop* cutefish::audio::AudioObject::getProp(const char*)':
    clients/audio/audio_object.cpp:34:34: error: 'spa_pod_find_prop' was not declared in this scope
       34 |     const spa_pod_prop* prop = spa_pod_find_prop(object, nullptr, key);
        |                                  ^~~~~~~~~~~
    clients/audio/audio_object.cpp:34:34: note: suggested alternative: 'spa_pod_for_each_prop'

Include <spa/pod/iter.h> explicitly to get the declaration.

clients/audio/audio_object.cpp

@@ -13,6 +13,7 @@
 #include <spa/param/props.h>
 #include <spa/param/route.h>
 #include <spa/pod/parser.h>
+#include <spa/pod/iter.h>
 
 #include "cutefish/audio/backend.h"