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/md.h b/include/mbedtls/md.h
index 6b6f5c5..bf29524 100644
--- a/include/mbedtls/md.h
+++ b/include/mbedtls/md.h
@@ -80,7 +80,8 @@
 /**
  * The generic message-digest context.
  */
-typedef struct {
+typedef struct mbedtls_md_context_t
+{
     /** Information about the associated message digest. */
     const mbedtls_md_info_t *md_info;