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/xtea.h b/include/mbedtls/xtea.h
index 8df708a..c70c3fe 100644
--- a/include/mbedtls/xtea.h
+++ b/include/mbedtls/xtea.h
@@ -50,7 +50,7 @@
 /**
  * \brief          XTEA context structure
  */
-typedef struct
+typedef struct mbedtls_xtea_context
 {
     uint32_t k[4];       /*!< key */
 }