suppress warning

* spec/rubyspec/optional/capi/ext/fixnum_spec.c: suppress
  unused-but-set-variable warning.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@58712 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

spec/rubyspec/optional/capi/ext/fixnum_spec.c

@@ -29,6 +29,8 @@ void Init_fixnum_spec(void) {
 #ifdef HAVE_RB_FIX2INT
   rb_define_method(cls, "rb_fix2int", fixnum_spec_rb_fix2int, 1);
 #endif
+
+  (void)cls;
 }
 
 #ifdef __cplusplus