Allow to forward declare of public structs #1215

Thanks to forward declare we can declare `struct` in our header file instead making #include

include/mbedtls/threading.h

@@ -42,7 +42,7 @@ extern "C" {
 
 #if defined(MBEDTLS_THREADING_PTHREAD)
 #include <pthread.h>
-typedef struct
+typedef struct mbedtls_threading_mutex_t
 {
     pthread_mutex_t mutex;
     char is_valid;