test_gem_ext_builder.rb: fix wrong skip condition

* test/rubygems/test_gem_ext_builder.rb (test_build_extensions_extconf_bad):
  fix wrong skip condition by removing wrong expace, String#include?
  does not turn a string argument into a regexp.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47201 b2dd03c8-39d4-4d8f-98ff-823fe69b080e