build: fix builds that disable protocols but not digest auth

- Build base64 functions if digest auth is not disabled.

Prior to this change if some protocols were disabled but not digest auth
then a build error would occur due to missing base64 functions.

Fixes https://github.com/curl/curl/issues/12440
Closes https://github.com/curl/curl/pull/12442

lib/base64.c

@@ -31,6 +31,7 @@
   !defined(CURL_DISABLE_SMTP) || \
   !defined(CURL_DISABLE_POP3) || \
   !defined(CURL_DISABLE_IMAP) || \
+  !defined(CURL_DISABLE_DIGEST_AUTH) || \
   !defined(CURL_DISABLE_DOH) || defined(USE_SSL) || defined(BUILDING_CURL)
 #include "curl/curl.h"
 #include "warnless.h"