MINOR: converters: add a "void *private" argument to converters

This permits to store specific configuration pointer. It is useful
with future Lua integration.
diff --git a/src/proto_http.c b/src/proto_http.c
index 4484bcf..3738aa4 100644
--- a/src/proto_http.c
+++ b/src/proto_http.c
@@ -11377,7 +11377,7 @@
  * adds an optional offset found in args[0] and emits a string representing
  * the date in RFC-1123/5322 format.
  */
-static int sample_conv_http_date(const struct arg *args, struct sample *smp)
+static int sample_conv_http_date(const struct arg *args, struct sample *smp, void *private)
 {
 	const char day[7][4] = { "Mon", "Tue", "Wed", "Thu", "Fri", "Sat", "Sun" };
 	const char mon[12][4] = { "Jan", "Feb", "Mar", "Apr", "May", "Jun", "Jul", "Aug", "Sep", "Oct", "Nov", "Dec" };
@@ -11428,7 +11428,7 @@
 }
 
 /* Arguments: The list of expected value, the number of parts returned and the separator */
-static int sample_conv_q_prefered(const struct arg *args, struct sample *smp)
+static int sample_conv_q_prefered(const struct arg *args, struct sample *smp, void *private)
 {
 	const char *al = smp->data.str.str;
 	const char *end = al + smp->data.str.len;