MINOR: http_ext: introduce http ext converters

This commit is really simple, it adds the required skeleton code to allow
new http_ext converter to be easily registered through STG_REGISTER facility.
diff --git a/src/http_ext.c b/src/http_ext.c
index 6a922e6..1c621a6 100644
--- a/src/http_ext.c
+++ b/src/http_ext.c
@@ -21,6 +21,8 @@
 #include <haproxy/sc_strm.h>
 #include <haproxy/obj_type.h>
 #include <haproxy/cfgparse.h>
+#include <haproxy/arg.h>
+#include <haproxy/initcall.h>
 #include <haproxy/tools.h>
 
 /* check if char is a valid obfuscated identifier char
@@ -1362,3 +1364,15 @@
 		dest->hdr_name = istdup(orig->hdr_name);
 	dest->except_net = orig->except_net;
 }
+
+/*
+ * =========== CONV ===========
+ * related converters
+ */
+
+/* Note: must not be declared <const> as its list will be overwritten */
+static struct sample_conv_kw_list sample_conv_kws = {ILH, {
+	{ NULL, NULL, 0, 0, 0 },
+}};
+
+INITCALL1(STG_REGISTER, sample_register_convs, &sample_conv_kws);