Fix mumble_ol.dll not being versioned under windows.

* Low impact fix for RC. Directly set VERSION in overlay.pro
* Currently the version is set multiple times
  in different .pro/pri files. This should be refactored to come from
  a single .pri file (similar to /macx/common.pri) used in all locations.
* Should probably use an RC file to be able to set more information

overlay/overlay.pro

@@ -1,5 +1,7 @@
 include (../compiler.pri)
 
+VERSION = 1.2.4
+TARGET_EXT = .dll
 TEMPLATE = lib
 CONFIG -= qt
 CONFIG *= dll shared debug_and_release warn_on