win32.c: fix infinite recursion

* win32/win32.c (rb_w32_pow): undef pow to get rid of infinite
  recursive call.  re-fix [Bug #8495].  [ruby-core:55923] [Bug #8621]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@41923 b2dd03c8-39d4-4d8f-98ff-823fe69b080e