MINOR: http/conf: store the use_backend configuration file and line for logs

The error log of the directive use_backend doesn't provide the
file and line containing the declaration. This patch stores
theses informations.
diff --git a/include/types/arg.h b/include/types/arg.h
index 7576f8a..651164b 100644
--- a/include/types/arg.h
+++ b/include/types/arg.h
@@ -77,6 +77,7 @@
 	ARGC_CAP,      /* capture rule */
 	ARGC_SRV,      /* server line */
 	ARGC_SPOE,     /* spoe message args */
+	ARGC_UBK,      /* use_backend message */
 };
 
 /* flags used when compiling and executing regex */