Allow to forward declare of public structs #1215

Thanks to forward declare we can declare `struct` in our header file instead making #include
diff --git a/include/mbedtls/net_sockets.h b/include/mbedtls/net_sockets.h
index 0f9b31e..40f3317 100644
--- a/include/mbedtls/net_sockets.h
+++ b/include/mbedtls/net_sockets.h
@@ -68,7 +68,7 @@
  * (eg two file descriptors for combined IPv4 + IPv6 support, or additional
  * structures for hand-made UDP demultiplexing).
  */
-typedef struct
+typedef struct mbedtls_net_context
 {
     int fd;             /**< The underlying file descriptor                 */
 }