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/des.h b/include/mbedtls/des.h
index 6eb7d03..91d16b6 100644
--- a/include/mbedtls/des.h
+++ b/include/mbedtls/des.h
@@ -61,7 +61,7 @@
  *                 security risk. We recommend considering stronger ciphers
  *                 instead.
  */
-typedef struct
+typedef struct mbedtls_des_context
 {
     uint32_t sk[32];            /*!<  DES subkeys       */
 }
@@ -70,7 +70,7 @@
 /**
  * \brief          Triple-DES context structure
  */
-typedef struct
+typedef struct mbedtls_des3_context
 {
     uint32_t sk[96];            /*!<  3DES subkeys      */
 }