staging: rtl8188eu: Fix build error when CFG80211 is not selected

The kbuild test robot reports the following build error for
i386-randconfig-c0-09230740:

>> ERROR: "ieee80211_hdrlen" [drivers/staging/rtl8188eu/r8188eu.ko] undefined!

Add a dependency on CFG80211 to fix it.
Signed-off-by: Jakub Sitnicki <jsitnicki@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/rtl8188eu/Kconfig

 config R8188EU
 	tristate "Realtek RTL8188EU Wireless LAN NIC driver"
-	depends on WLAN && USB
+	depends on WLAN && USB && CFG80211
 	select WIRELESS_EXT
 	select WEXT_PRIV
 	---help---