Fix rb_define_singleton_method warning

for debug counters

```
../include/ruby/intern.h:1175:137: warning: passing argument 3 of 'rb_define_singleton_method0' from incompatible pointer type [-Wincompatible-pointer-types]
 #define rb_define_singleton_method(klass, mid, func, arity) rb_define_singleton_method_choose_prototypem3((arity),(func))((klass),(mid),(func),(arity));
                                                                                                                                         ^
../vm.c:2958:5: note: in expansion of macro 'rb_define_singleton_method'
     rb_define_singleton_method(rb_cRubyVM, "show_debug_counters", rb_debug_counter_show, 0);
     ^~~~~~~~~~~~~~~~~~~~~~~~~~
../include/ruby/intern.h:1139:99: note: expected 'VALUE (*)(VALUE) {aka long unsigned int (*)(long unsigned int)}' but argument is of type 'VALUE (*)(void) {aka long unsigned int (*)(void)}'
 __attribute__((__unused__,__weakref__("rb_define_singleton_method"),__nonnull__(2,3)))static void rb_define_singleton_method0 (VALUE,const char*,VALUE(*)(VALUE),int);
```

debug_counter.c

@@ -44,14 +44,14 @@ rb_debug_counter_show_results(const char *msg)
 }
 
 VALUE
-rb_debug_counter_show(void)
+rb_debug_counter_show(RB_UNUSED_VAR(VALUE klass))
 {
     rb_debug_counter_show_results("method call");
     return Qnil;
 }
 
 VALUE
-rb_debug_counter_reset(void)
+rb_debug_counter_reset(RB_UNUSED_VAR(VALUE klass))
 {
     for (int i = 0; i < RB_DEBUG_COUNTER_MAX; i++) {
         switch (i) {

debug_counter.h

@@ -349,8 +349,8 @@ rb_debug_counter_add(enum rb_debug_counter_type type, int add, int cond)
     return cond;
 }
 
-VALUE rb_debug_counter_reset(void);
-VALUE rb_debug_counter_show(void);
+VALUE rb_debug_counter_reset(VALUE klass);
+VALUE rb_debug_counter_show(VALUE klass);
 
 #define RB_DEBUG_COUNTER_INC(type)                rb_debug_counter_add(RB_DEBUG_COUNTER_##type, 1, 1)
 #define RB_DEBUG_COUNTER_INC_UNLESS(type, cond) (!rb_debug_counter_add(RB_DEBUG_COUNTER_##type, 1, !(cond)))