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/ripemd160.h b/include/mbedtls/ripemd160.h
index a0dac0c..0c8e568 100644
--- a/include/mbedtls/ripemd160.h
+++ b/include/mbedtls/ripemd160.h
@@ -46,7 +46,7 @@
 /**
  * \brief          RIPEMD-160 context structure
  */
-typedef struct
+typedef struct mbedtls_ripemd160_context
 {
     uint32_t total[2];          /*!< number of bytes processed  */
     uint32_t state[5];          /*!< intermediate digest state  */