grab PAGE_SHIFT from a location where it is defined nowadays

--- a/machine/m_linux.c
+++ b/machine/m_linux.c
@@ -68,7 +68,7 @@
 #include <sys/vfs.h>
 
 #include <sys/param.h>		/* for HZ */
-#include <asm/page.h>		/* for PAGE_SHIFT */
+#include <sys/user.h>		/* for PAGE_SHIFT */
 
 #if 0
 #include <linux/proc_fs.h>	/* for PROC_SUPER_MAGIC */