Fix to fail searching libxml2/libxslt on mswin.

ext/nokogiri/extconf.rb searches xml2.lib, xslt.lib and exslt.lib on mswin platform.
But it fails because libxml2 and libxslt installs libxml2.lib, libxslt.lib and libexslt.lib.

I tested with:
- Windows 7 SP1
- cl.exe for x64 (Visual Studio 2013 Express for Windows Desktop)
- Ruby 2.1.0 (ruby 2.1.0p0 (2013-12-25 revision 44422) [x64-mswin64_120])
- libxml2 2.8.0
- libxslt 1.1.28