spell: use "const char *" variables for result of string functions with const parameter

src/plugins/spell/spell-command.c

@@ -140,7 +140,8 @@ void
 spell_command_speller_list_dicts (void)
 {
 #ifndef USE_ENCHANT
-    char *country, *lang, *pos, *iso;
+    const char *pos;
+    char *country, *lang, *iso;
     char str_dict[256], str_country[128];
     struct AspellConfig *config;
     AspellDictInfoList *list;

src/plugins/spell/spell.c

@@ -391,7 +391,7 @@ spell_string_is_url (const char *word)
  */
 
 int
-spell_string_is_nick (struct t_gui_buffer *buffer, const char *word)
+spell_string_is_nick (struct t_gui_buffer *buffer, char *word)
 {
     char *pos, *pos_nick_completer, *pos_space, saved_char;
     const char *buffer_type, *buffer_nick, *buffer_channel;