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/pkcs11.h b/include/mbedtls/pkcs11.h
index bf65c55..02427dd 100644
--- a/include/mbedtls/pkcs11.h
+++ b/include/mbedtls/pkcs11.h
@@ -50,7 +50,8 @@
 /**
  * Context for PKCS #11 private keys.
  */
-typedef struct {
+typedef struct mbedtls_pkcs11_context
+{
         pkcs11h_certificate_t pkcs11h_cert;
         int len;
 } mbedtls_pkcs11_context;