bmcweb: Add X-Content-Type-Options header

Because we should always be posting the correct mime type on all of our
pages, add a "nosniff" flag, to improve our protection against injection
attacks.

https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/X-Content-Type-Options



Change-Id: I4a311948e5e157b4071effddff577c73b65d7af1
Signed-off-by: Ed Tanous <ed.tanous@intel.com>