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/entropy.h b/include/mbedtls/entropy.h
index a5cb05a..ca06dc3 100644
--- a/include/mbedtls/entropy.h
+++ b/include/mbedtls/entropy.h
@@ -107,7 +107,7 @@
 /**
  * \brief           Entropy source state
  */
-typedef struct
+typedef struct mbedtls_entropy_source_state
 {
     mbedtls_entropy_f_source_ptr    f_source;   /**< The entropy source callback */
     void *          p_source;   /**< The callback data pointer */
@@ -120,7 +120,7 @@
 /**
  * \brief           Entropy context structure
  */
-typedef struct
+typedef struct mbedtls_entropy_context
 {
     int accumulator_started;
 #if defined(MBEDTLS_ENTROPY_SHA512_ACCUMULATOR)