Add missing include

The file  Fireworks/Core/src/CmsShowMain.cc needs an include of X11/Xlib.h in order to compile.  THis include is missing.  In ROOT 5, the include is included indirectly through the ROOT header TGX11.h. In ROOT 6, TGX11.h does not include X11/Xlib.h, and CmsShowMain.cc does not compile.  This pull request adds the missing include.