Fix a compile error on zero mrbgem case.

tasks/mrbgems.rake

@@ -34,11 +34,13 @@ MRuby.each_target do
         f.puts %Q[#include <mruby.h>]
         f.puts %Q[]
         f.write gem_func_decls
+        unless gem_final_calls.empty?
         f.puts %Q[]
-        f.puts %Q[static void]
-        f.puts %Q[mrb_final_mrbgems(mrb_state *mrb) {]
-        f.write gem_final_calls
-        f.puts %Q[}]
+          f.puts %Q[static void]
+          f.puts %Q[mrb_final_mrbgems(mrb_state *mrb) {]
+          f.write gem_final_calls
+          f.puts %Q[}]
+        end
         f.puts %Q[]
         f.puts %Q[void]
         f.puts %Q[mrb_init_mrbgems(mrb_state *mrb) {]