Give all anonymously typedeffed structs in headers a name

Anonymous structs cannot be forward declared and have no benefit.

libavcodec/w32pthreads.h

@@ -42,7 +42,7 @@
 #include "libavutil/internal.h"
 #include "libavutil/mem.h"
 
-typedef struct {
+typedef struct pthread_t {
     void *handle;
     void *(*func)(void* arg);
     void *arg;
@@ -56,7 +56,7 @@ typedef CRITICAL_SECTION pthread_mutex_t;
 /* This is the CONDITIONAL_VARIABLE typedef for using Window's native
  * conditional variables on kernels 6.0+.
  * MinGW does not currently have this typedef. */
-typedef struct {
+typedef struct pthread_cond_t {
     void *ptr;
 } pthread_cond_t;
 
@@ -117,7 +117,7 @@ static inline int pthread_mutex_unlock(pthread_mutex_t *m)
 
 /* for pre-Windows 6.0 platforms we need to define and use our own condition
  * variable and api */
-typedef struct {
+typedef struct  win32_cond_t {
     pthread_mutex_t mtx_broadcast;
     pthread_mutex_t mtx_waiter_count;
     volatile int waiter_count;