OS/2 TB breakage

Uh duh. It would help if I put the XP_OS2 in the header file.

gfx/public/nsIFontMetrics.h

@@ -206,7 +206,7 @@ public:
    */
   NS_IMETHOD  GetFontHandle(nsFontHandle &aHandle) = 0;
 
-#ifdef _WIN32
+#if defined(_WIN32) || defined(XP_OS2)
   /**
    * Returns the average character width
    */