staging: rtl8188eu: Add missing include to pick up prototype

This fixes the following sparse warning:

drivers/staging/rtl8188eu/hal/HalHWImg8188E_MAC.c:119:6: warning: symbol
'rtl88e_phy_mac_config' was not declared. Should it be static?

Signed-off-by: Max Vozeler <max@hinterhof.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>