Fixed warning (ros2#1350)

Signed-off-by: Alejandro Hernandez Cordero <ahcorde@gmail.com>

Author: ahcorde

rcl/src/rcl/expand_topic_name.c

@@ -136,10 +136,10 @@ rcl_expand_topic_name(
     //
     // These assumptions are taken because this is checked in the validation function.
     const char * current_output = (local_output) ? local_output : input_topic_name;
-    char * next_opening_brace = NULL;
+    const char * next_opening_brace = NULL;
     // current_output may be replaced on each loop if a substitution is made
     while ((next_opening_brace = strchr(current_output, '{')) != NULL) {
-      char * next_closing_brace = strchr(current_output, '}');
+      const char * next_closing_brace = strchr(current_output, '}');
       // conclusion based on above assumptions: next_closing_brace - next_opening_brace > 1
       size_t substitution_substr_len = next_closing_brace - next_opening_brace + 1;
       // figure out what the replacement is for this substitution