Add support for readline 8.2

In readline 8.2 the type of rl_completer_word_break_characters changed to
include const.
diff --git a/gdb/completer.c b/gdb/completer.c
index d3900ae..a51c16a 100644
--- a/gdb/completer.c
+++ b/gdb/completer.c
@@ -36,7 +36,7 @@
    calling a hook instead so we eliminate the CLI dependency.  */
 #include "gdbcmd.h"
 
-/* Needed for rl_completer_word_break_characters() and for
+/* Needed for rl_completer_word_break_characters and for
    rl_filename_completion_function.  */
 #include "readline/readline.h"
 
@@ -2011,7 +2011,7 @@ gdb_completion_word_break_characters_throw ()
       rl_basic_quote_characters = NULL;
     }
 
-  return rl_completer_word_break_characters;
+  return (char *) rl_completer_word_break_characters;
 }
 
 char *