build: remove video_mixer.h dependency on some headers

This dependency causes a almost complet build if only video_mixer
is changed.

Issue: #81174
Change-Id: I1d374b582e6f388962ff78f67ee03b967ee82779

src/conference.cpp

@@ -30,6 +30,7 @@
 #include "sip/sipcall.h"
 #include "client/videomanager.h"
 #include "video/video_input.h"
+#include "video/video_mixer.h"
 #endif
 
 #include "call_factory.h"

src/conference.h

@@ -30,12 +30,14 @@
 
 #include "audio/recordable.h"
 
+namespace ring {
+
 #ifdef RING_VIDEO
-#include "video/video_mixer.h"
+namespace video {
+class VideoMixer;
+}
 #endif
 
-namespace ring {
-
 typedef std::set<std::string> ParticipantSet;
 
 class Conference : public Recordable {

src/media/video/video_sender.h

@@ -25,7 +25,6 @@
 #include "noncopyable.h"
 #include "media_encoder.h"
 #include "media_io_handle.h"
-#include "video_mixer.h"
 
 #include <map>
 #include <string>