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/rsa.h b/include/mbedtls/rsa.h
index 19eb2ee..6eea5af 100644
--- a/include/mbedtls/rsa.h
+++ b/include/mbedtls/rsa.h
@@ -92,7 +92,7 @@
  *          is deprecated. All manipulation should instead be done through
  *          the public interface functions.
  */
-typedef struct
+typedef struct mbedtls_rsa_context
 {
     int ver;                    /*!<  Always 0.*/
     size_t len;                 /*!<  The size of \p N in Bytes. */