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/sha1.h b/include/mbedtls/sha1.h
index 65a124c..7a19da0 100644
--- a/include/mbedtls/sha1.h
+++ b/include/mbedtls/sha1.h
@@ -58,7 +58,7 @@
  *                 stronger message digests instead.
  *
  */
-typedef struct
+typedef struct mbedtls_sha1_context
 {
     uint32_t total[2];          /*!< The number of Bytes processed.  */
     uint32_t state[5];          /*!< The intermediate digest state.  */