Use const void* instead of const char*

Use cast from `char*` to `void*` instead of union in opendir_without_gvl,
because convert from `void*` to `char*` without union in nogvl_opendir.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@60875 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

dir.c

@@ -504,7 +504,7 @@ static DIR *
 opendir_without_gvl(const char *path)
 {
     if (vm_initialized) {
-	union { const char *in; void *out; } u;
+	union { const void *in; void *out; } u;
 
 	u.in = path;