benchmark/app_aobench.rb: add `srand(0)`

To prevent noise for benchmark result.  Just for the case.
[Bug #15552]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@66893 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

benchmark/app_aobench.rb

@@ -282,6 +282,8 @@ alias printf_orig printf
 def printf *args
 end
 
+srand(0)
+
 # File.open("ao.ppm", "w") do |fp|
   printf("P6\n")
   printf("%d %d\n", IMAGE_WIDTH, IMAGE_HEIGHT)