Move definition to cipher.h
Define `MBEDTLS_CIPHER_MODE_STREAM` for `MBEDTLS_CIPHER_NULL_CIPHER`
as well, in cipher.h. Remove redundant definition in `cipher_internal.h`
diff --git a/include/mbedtls/cipher.h b/include/mbedtls/cipher.h
index a1f4738..ea0ce98 100644
--- a/include/mbedtls/cipher.h
+++ b/include/mbedtls/cipher.h
@@ -45,7 +45,7 @@
#define MBEDTLS_CIPHER_MODE_WITH_PADDING
#endif
-#if defined(MBEDTLS_ARC4_C)
+#if defined(MBEDTLS_ARC4_C) || defined(MBEDTLS_CIPHER_NULL_CIPHER)
#define MBEDTLS_CIPHER_MODE_STREAM
#endif