[MINOR] acl: add support for matching of RDP cookies

The RDP protocol is quite simple and documented, which permits
an easy detection and extraction of cookies. It can be useful
to match the MSTS cookie which can contain the username specified
by the client.
diff --git a/src/acl.c b/src/acl.c
index b26a527..f86b1b0 100644
--- a/src/acl.c
+++ b/src/acl.c
@@ -810,6 +810,7 @@
 	{ .name = "HTTP_URL_SLASH", .expr = {"url_beg","/",""}},
 	{ .name = "HTTP_URL_STAR",  .expr = {"url","*",""}},
 	{ .name = "HTTP_CONTENT",   .expr = {"hdr_val(content-length)","gt","0",""}},
+	{ .name = "RDP_COOKIE",     .expr = {"req_rdp_cookie_cnt","gt","0",""}},
 	{ .name = "REQ_CONTENT",    .expr = {"req_len","gt","0",""}},
 	{ .name = "WAIT_END",       .expr = {"wait_end",""}},
 	{ .name = NULL, .expr = {""}}