BUILD: do not specify "const" on functions returning structs or scalars

Older compilers (like gcc-3.4) warn about the use of "const" on functions
returning a struct, which makes sense since the return may only be copied :

  include/common/htx.h:233: warning: type qualifiers ignored on function return type

Let's simply drop "const" here.
diff --git a/include/common/http.h b/include/common/http.h
index 2f7486e..b0befa5 100644
--- a/include/common/http.h
+++ b/include/common/http.h
@@ -136,7 +136,7 @@
 
 int init_http(char **err);
 enum http_meth_t find_http_meth(const char *str, const int len);
-const int http_get_status_idx(unsigned int status);
+int http_get_status_idx(unsigned int status);
 const char *http_get_reason(unsigned int status);
 struct ist http_get_path(const struct ist uri);
 int http_header_match2(const char *hdr, const char *end,