MINOR: log: One const should be enough.

"const const" doesn't bring much more constness, so only use one.
diff --git a/include/proto/log.h b/include/proto/log.h
index 7fb1f04..b994470 100644
--- a/include/proto/log.h
+++ b/include/proto/log.h
@@ -151,7 +151,7 @@
  *
  * Return the adress of the \0 character, or NULL on error
  */
-char *lf_text_len(char *dst, const const char *src, size_t len, size_t size, const struct logformat_node *node);
+char *lf_text_len(char *dst, const char *src, size_t len, size_t size, const struct logformat_node *node);
 
 /*
  * Write a IP adress to the log string