tcltklib.c: GC guard

* ext/tk/tcltklib.c (setup_rubytkkit): add GC guard instead of
  volatile.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@52401 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

ext/tk/tcltklib.c

@@ -1263,10 +1263,11 @@ setup_rubytkkit(void)
 #ifdef __WIN32__
     /* rbtk_win32_SetHINSTANCE("tcltklib.so"); */
     {
-      volatile VALUE basename;
+      VALUE basename;
       basename = rb_funcall(rb_cFile, rb_intern("basename"), 1,
 			    rb_str_new2(rb_sourcefile()));
       rbtk_win32_SetHINSTANCE(RSTRING_PTR(basename));
+      RB_GC_GUARD(basename);
     }
 #endif
     set_rubytk_kitpath(rb_sourcefile());