base64: enable in build with SMTP

The oauth2 support is used with SMTP and it uses base64 functions.

Reported-by: Michael Olbrich
Fixes #5937
Closes #5938

lib/base64.c

@@ -5,7 +5,7 @@
  *                            | (__| |_| |  _ <| |___
  *                             \___|\___/|_| \_\_____|
  *
- * Copyright (C) 1998 - 2019, Daniel Stenberg, <daniel@haxx.se>, et al.
+ * Copyright (C) 1998 - 2020, Daniel Stenberg, <daniel@haxx.se>, et al.
  *
  * This software is licensed as described in the file COPYING, which
  * you should have received as part of this distribution. The terms
@@ -26,6 +26,7 @@
 
 #if !defined(CURL_DISABLE_HTTP_AUTH) || defined(USE_SSH) || \
   !defined(CURL_DISABLE_LDAP) || \
+  !defined(CURL_DISABLE_SMTP) || \
   !defined(CURL_DISABLE_DOH) || defined(USE_SSL)
 
 #include "urldata.h" /* for the Curl_easy definition */