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/ctr_drbg.h b/include/mbedtls/ctr_drbg.h
index 3835d72..3a4b7f3 100644
--- a/include/mbedtls/ctr_drbg.h
+++ b/include/mbedtls/ctr_drbg.h
@@ -108,7 +108,7 @@
 /**
  * \brief          The CTR_DRBG context structure.
  */
-typedef struct
+typedef struct mbedtls_ctr_drbg_context
 {
     unsigned char counter[16];  /*!< The counter (V). */
     int reseed_counter;         /*!< The reseed counter. */