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/cipher.h b/include/mbedtls/cipher.h
index ea0ce98..3ac17f6 100644
--- a/include/mbedtls/cipher.h
+++ b/include/mbedtls/cipher.h
@@ -235,7 +235,8 @@
  * Cipher information. Allows calling cipher functions
  * in a generic way.
  */
-typedef struct {
+typedef struct mbedtls_cipher_info_t
+{
     /** Full cipher identifier. For example,
      * MBEDTLS_CIPHER_AES_256_CBC.
      */
@@ -276,7 +277,8 @@
 /**
  * Generic cipher context.
  */
-typedef struct {
+typedef struct mbedtls_cipher_context_t
+{
     /** Information about the associated cipher. */
     const mbedtls_cipher_info_t *cipher_info;