Include proper header file for PFN_DOWN()

The recent commit (99a10a60) to fix up
mmap_kmem() broke compiles because it used PFN_DOWN() without including
<linux/pfn.h>.
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>

drivers/char/mem.c

@@ -26,6 +26,7 @@
 #include <linux/backing-dev.h>
 #include <linux/bootmem.h>
 #include <linux/pipe_fs_i.h>
+#include <linux/pfn.h>
 
 #include <asm/uaccess.h>
 #include <asm/io.h>