[Win] Unreviewed buildfix after r150161.

'Unknown' in HTTPRequest.cpp is ambiguous symbol with MSVC,
because it is also defined in MS SDK 7.1 winioctl.h.

* UIProcess/InspectorServer/HTTPRequest.cpp:
(WebKit::HTTPRequest::HTTPRequest):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@150179 268f45cc-cd09-0410-ab3c-d52691b4dbfc