win: fix 'floor' undefined compiler warning

In VS2010 debug build, output message is `libuv\src\win\util.c(590):
warning C4013: 'floor' undefined; assuming extern returning int`.

PR-URL: libuv#1814
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

src/win/util.c

@@ -37,7 +37,7 @@
 #include <tlhelp32.h>
 #include <windows.h>
 #include <userenv.h>
-
+#include <math.h>
 
 /*
  * Max title length; the only thing MSDN tells us about the maximum length