smtp: Moved auth-mechanism constants into a separate header file

Move the SMTP_AUTH constants into a separate header file in
preparation for adding SASL based authentication to POP3 as the two
protocols will need to share them.

lib/smtp.c

@@ -88,6 +88,7 @@
 #include "curl_hmac.h"
 #include "curl_gethostname.h"
 #include "curl_ntlm_msgs.h"
+#include "curl_sasl.h"
 #include "warnless.h"
 
 #define _MPRINTF_REPLACE /* use our functions only */