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/aria.h b/include/mbedtls/aria.h
index bae0621..4a79c13 100644
--- a/include/mbedtls/aria.h
+++ b/include/mbedtls/aria.h
@@ -62,7 +62,7 @@
 /**
  * \brief The ARIA context-type definition.
  */
-typedef struct
+typedef struct mbedtls_aria_context
 {
     unsigned char nr;           /*!< The number of rounds (12, 14 or 16) */
     /*! The ARIA round keys. */