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/havege.h b/include/mbedtls/havege.h
index d4cb3ed..57e8c40 100644
--- a/include/mbedtls/havege.h
+++ b/include/mbedtls/havege.h
@@ -35,7 +35,7 @@
 /**
  * \brief          HAVEGE state structure
  */
-typedef struct
+typedef struct mbedtls_havege_state
 {
     int PT1, PT2, offset[2];
     int pool[MBEDTLS_HAVEGE_COLLECT_SIZE];