of: include linux/proc_fs.h

We use a few procfs-specific functions (eg, proc_device_tree_*) which
aren't covered by the current includes. This causes the following build
error on arm:

drivers/of/base.c: In function 'prom_add_property':
drivers/of/base.c:861: error: implicit declaration of function 'proc_device_tree_add_prop'
drivers/of/base.c: In function 'prom_remove_property':
drivers/of/base.c:902: error: implicit declaration of function 'proc_device_tree_remove_prop'
drivers/of/base.c: In function 'prom_update_property':
drivers/of/base.c:946: error: implicit declaration of function 'proc_device_tree_update_prop'

Add proc_fs.h for these prototypes.

Signed-off-by: Jeremy Kerr <jeremy.kerr@canonical.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>