base64: also build for pop3 and imap

Follow-up to the fix in 20417a13fb

Reported-by: Michael Olbrich
Fixes #5937
Closes #5948

lib/base64.c

@@ -27,6 +27,8 @@
 #if !defined(CURL_DISABLE_HTTP_AUTH) || defined(USE_SSH) || \
   !defined(CURL_DISABLE_LDAP) || \
   !defined(CURL_DISABLE_SMTP) || \
+  !defined(CURL_DISABLE_POP3) || \
+  !defined(CURL_DISABLE_IMAP) || \
   !defined(CURL_DISABLE_DOH) || defined(USE_SSL)
 
 #include "urldata.h" /* for the Curl_easy definition */