Fix function prototypes for list of arguments

At the moment, building WeeChat triggers several thousand -Wstrict-prototypes
diagnostics.  This is due to its source code using an empty argument list for
functions and function pointers that take no arguments, instead of explicitly
declaring that they take no arguments by using a void list.

This commit replaces all empty argument lists with a void list.

Note that Ruby's headers also suffer the same problem, which WeeChat can't
do anything to fix.  Thus, building WeeChat with the Ruby plugin enabled
will still issue approximately 30 such diagnostics.

src/plugins/spell/spell-command.c

@@ -135,7 +135,7 @@ spell_enchant_dict_describe_cb (const char *lang_tag,
  */
 
 void
-spell_command_speller_list_dicts ()
+spell_command_speller_list_dicts (void)
 {
 #ifndef USE_ENCHANT
     char *country, *lang, *pos, *iso;
@@ -478,7 +478,7 @@ spell_command_cb (const void *pointer, void *data,
  */
 
 void
-spell_command_init ()
+spell_command_init (void)
 {
     weechat_hook_command (
         "spell",