RHEL 7: curl: Fix plugin for older curl which lacks SMB support.

RHEL 7 has curl-7.29.0-51.el7 which lacks SMB support so you would see
the error below.  These are #defines in newer curl so we can simply
make the code conditional.

curl.c:121:12: error: 'CURLPROTO_SMB' undeclared here (not in a function)
   { "smb", CURLPROTO_SMB },
            ^
curl.c:122:13: error: 'CURLPROTO_SMBS' undeclared here (not in a function)
   { "smbs", CURLPROTO_SMBS },
             ^

plugins/curl/curl.c

@@ -118,8 +118,12 @@ static struct { const char *name; long bitmask; } curl_protocols[] = {
   { "rtmps", CURLPROTO_RTMPS },
   { "rtmpts", CURLPROTO_RTMPTS },
   { "gopher", CURLPROTO_GOPHER },
+#ifdef CURLPROTO_SMB
   { "smb", CURLPROTO_SMB },
+#endif
+#ifdef CURLPROTO_SMBS
   { "smbs", CURLPROTO_SMBS },
+#endif
   { NULL }
 };