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/aes.h b/include/mbedtls/aes.h
index f6603d5..4c8dab3 100644
--- a/include/mbedtls/aes.h
+++ b/include/mbedtls/aes.h
@@ -79,7 +79,7 @@
 /**
  * \brief The AES context-type definition.
  */
-typedef struct
+typedef struct mbedtls_aes_context
 {
     int nr;                     /*!< The number of rounds. */
     uint32_t *rk;               /*!< AES round keys. */
@@ -98,7 +98,7 @@
 /**
  * \brief The AES XTS context-type definition.
  */
-typedef struct
+typedef struct mbedtls_aes_xts_context
 {
     mbedtls_aes_context crypt; /*!< The AES context to use for AES block
                                         encryption or decryption. */