MINOR: log: add lf_text_len

This function allows to log a text of a specific length.
diff --git a/include/proto/log.h b/include/proto/log.h
index 5d1215a..173375e 100644
--- a/include/proto/log.h
+++ b/include/proto/log.h
@@ -122,7 +122,7 @@
  *
  * Return the adress of the \0 character, or NULL on error
  */
-char *lf_text(char *dst, const char *src, size_t size, struct logformat_node *node);
+char *lf_text_len(char *dst, const char *src, size_t len, size_t size, struct logformat_node *node);
 
 /*
  * Write a IP adress to the log string