remove compiler warnings.

tasks/mrbgems_test.rake

@@ -19,10 +19,10 @@ MRuby.each_target do
         end
         f.puts %Q[void mrb_#{g.funcname}_gem_test(mrb_state *mrb);] unless g.test_objs.empty?
         f.puts %Q[void GENERATED_TMP_mrb_#{g.funcname}_gem_test(mrb_state *mrb) {]
-        f.puts %Q[  mrb_state *mrb2;]
-        f.puts %Q[  mrb_value val1, val2, ary1, ary2;]
-        f.puts %Q[  int ai;]
         unless g.test_rbfiles.empty?
+          f.puts %Q[  mrb_state *mrb2;]
+          f.puts %Q[  mrb_value val1, val2, ary1, ary2;]
+          f.puts %Q[  int ai;]
           g.test_rbfiles.count.times do |i|
             f.puts %Q[  ai = mrb_gc_arena_save(mrb);]
             f.puts %Q[  mrb2 = mrb_open();]