Sun bug 6529003: Xorg should not be including <sys/immu.h> on Solaris

<sys/immu.h> was removed from the latest Solaris Nevada build, but it's
been useless to Xorg for a long time (it only declared a couple of kernel
variables)
<http://bugs.opensolaris.org/view_bug.do?bug_id=6529003>

hw/xfree86/os-support/xf86_OSlib.h

@@ -130,8 +130,8 @@ typedef signed long xf86ssize_t;
 # include <errno.h>
 
 # if defined(_NEED_SYSI86)
-#  include <sys/immu.h>
 #  if !(defined (sun) && defined (SVR4))
+#    include <sys/immu.h>
 #    include <sys/region.h>
 #  endif
 #  include <sys/proc.h>