Use AVOnce as a static variable consistently

Using AVOnce as a stack variable makes no sense as the state is lost
when the function exits.

This fixes repeated calls to av(filter/device)_register_all

libavdevice/alldevices.c

@@ -73,7 +73,7 @@ static void register_all(void)
 
 void avdevice_register_all(void)
 {
-    AVOnce control = AV_ONCE_INIT;
+    static AVOnce control = AV_ONCE_INIT;
 
     ff_thread_once(&control, register_all);
 }

libavfilter/allfilters.c

@@ -390,7 +390,7 @@ static void register_all(void)
 
 void avfilter_register_all(void)
 {
-    AVOnce control = AV_ONCE_INIT;
+    static AVOnce control = AV_ONCE_INIT;
 
     ff_thread_once(&control, register_all);
 }

libavformat/allformats.c

@@ -385,7 +385,7 @@ static void register_all(void)
 
 void av_register_all(void)
 {
-    AVOnce control = AV_ONCE_INIT;
+    static AVOnce control = AV_ONCE_INIT;
 
     ff_thread_once(&control, register_all);
 }