Do not treat retguard symbols as leaked.

Index: tool/leaked-globals
--- tool/leaked-globals.orig
+++ tool/leaked-globals
@@ -70,6 +70,7 @@ IO.foreach("|#{NM} #{ARGV.join(' ')}") do |line|
   next unless n.sub!(/^#{SYMBOL_PREFIX}/o, "")
   next if n.include?(".")
   next if !so and n.start_with?("___asan_")
+  next if !so and n.start_with?("__retguard_")
   case n
   when /\A(?:Init_|InitVM_|pm_|[Oo]nig|dln_|coroutine_)/
     next
