os: xprintf: fix includes

It's better coding style to include the header with prototypes for
our own functions directly, instead of relying on it being included
indirectly.

Signed-off-by: Enrico Weigelt, metux IT consult <info@metux.net>

os/xprintf.c

@@ -59,12 +59,14 @@
 #include <dix-config.h>
 
 #include <X11/Xos.h>
-#include "os.h"
 #include <stdarg.h>
 #include <stdio.h>
 #include <errno.h>
 #include <string.h>
 
+#include "include/os.h"
+#include "include/Xprintf.h"
+
 /**
  * Varargs sprintf that allocates a string buffer the right size for
  * the pattern & data provided and prints the requested data to it.