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/blowfish.h b/include/mbedtls/blowfish.h
index 985faa4..eea6882 100644
--- a/include/mbedtls/blowfish.h
+++ b/include/mbedtls/blowfish.h
@@ -55,7 +55,7 @@
 /**
  * \brief          Blowfish context structure
  */
-typedef struct
+typedef struct mbedtls_blowfish_context
 {
     uint32_t P[MBEDTLS_BLOWFISH_ROUNDS + 2];    /*!<  Blowfish round keys    */
     uint32_t S[4][256];                 /*!<  key dependent S-boxes  */