MINOR: spoe: Make the SPOE filter compatible with HTX proxies

There is any specific HTTP processing in the SPOE. So there is no reason to not
use it on HTX proxies.

This patch may be backported to 1.9.

src/flt_spoe.c

@@ -2998,6 +2998,7 @@ spoe_init(struct proxy *px, struct flt_conf *fconf)
 		sighandler_registered = 1;
 	}
 
+	fconf->flags |= FLT_CFG_FL_HTX;
 	return 0;
 }