Add dixAllocatePrivate stub to dummylib for utils

Normally not necessary, except when building non-optimized/debug causes
the inline functions from private.h to appear in os-support/libxorgos.la

hw/xfree86/dummylib/Makefile.am

@@ -13,6 +13,7 @@ STRL_SRCS = $(top_srcdir)/os/strlcat.c $(top_srcdir)/os/strlcpy.c
 endif
 
 libdummy_nonserver_a_SOURCES = \
+	dixprivates.c \
 	fatalerror.c \
 	logvwrite.c \
 	$(STRL_SRCS) \

hw/xfree86/dummylib/dixprivates.c

@@ -0,0 +1,19 @@
+#ifdef HAVE_XORG_CONFIG_H
+#include <xorg-config.h>
+#endif
+
+#include <X11/X.h>
+#include "os.h"
+#include "xf86.h"
+#include "xf86Priv.h"
+
+/*
+ * Utility functions required by libxf86_os. 
+ */
+
+_X_EXPORT pointer *
+dixAllocatePrivate(PrivateRec **privates, const DevPrivateKey key)
+{
+    return NULL;	/* not used */
+}
+