Give all anonymously typedeffed structs in headers a name

Anonymous structs cannot be forward declared and have no benefit.

libavformat/mms.h

@@ -23,11 +23,11 @@
 
 #include "url.h"
 
-typedef struct {
+typedef struct MMSStream {
     int id;
 }MMSStream;
 
-typedef struct {
+typedef struct MMSContext {
     URLContext *mms_hd;                  ///< TCP connection handle
     MMSStream *streams;