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/md2.h b/include/mbedtls/md2.h
index 08e75b2..a46bddb 100644
--- a/include/mbedtls/md2.h
+++ b/include/mbedtls/md2.h
@@ -55,7 +55,7 @@
  *                 stronger message digests instead.
  *
  */
-typedef struct
+typedef struct mbedtls_md2_context
 {
     unsigned char cksum[16];    /*!< checksum of the data block */
     unsigned char state[48];    /*!< intermediate digest state  */