windows: enable watching signals with process.on('SIGXYZ')

This reverts commit ea1cba6.

The offending commit was intended to land on the v0.8 branch only, but
it accidentally got merged at some point.

Closes nodejs#5054.

src/node.js

@@ -733,10 +733,6 @@
   };
 
   startup.processSignalHandlers = function() {
-    // Not supported on Windows.
-    if (process.platform === 'win32')
-      return;
-
     // Load events module in order to access prototype elements on process like
     // process.addListener.
     var signalWraps = {};