Allow to forward declare of public structs #1215

Thanks to forward declare we can declare `struct` in our header file instead making #include

include/mbedtls/sha512.h

@@ -52,7 +52,7 @@ extern "C" {
  *                 checksum calculations. The choice between these two is
  *                 made in the call to mbedtls_sha512_starts_ret().
  */
-typedef struct
+typedef struct mbedtls_sha512_context
 {
     uint64_t total[2];          /*!< The number of Bytes processed. */
     uint64_t state[8];          /*!< The intermediate digest state. */