staging: wean rtl8712 off of its ancient duplicate of ip.h

This driver should not be carrying around ancient copies of
headers like <linux/ip.h> for its own use.  Mapping it onto
the mainline one uncovers no build issues.

Cc: Larry Finger <Larry.Finger@lwfinger.net>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>