MINOR: tcp: prepare support for the "capture" action

A few minor entries will be needed to capture sample fetches in requests
or responses. This patch just prepares the code for this.
diff --git a/src/sample.c b/src/sample.c
index 66c9a9c..9f22ef9 100644
--- a/src/sample.c
+++ b/src/sample.c
@@ -971,6 +971,7 @@
 		case ARGC_HRS: where = "in http-response header format string in"; break;
 		case ARGC_UIF: where = "in unique-id-format string in"; break;
 		case ARGC_RDR: where = "in redirect format string in"; break;
+		case ARGC_CAP: where = "in capture rule in"; break;
 		case ARGC_ACL: ctx = "ACL keyword"; break;
 		}