staging: rtl8723bs: select CONFIG_CRYPTO_LIB_ARC4

The other rtlwifi drivers already have this, but r8723bs
was converted to the generic implementation without adding
the select:

ERROR: modpost: "arc4_crypt" [drivers/staging/rtl8723bs/r8723bs.ko] undefined!
ERROR: modpost: "arc4_setkey" [drivers/staging/rtl8723bs/r8723bs.ko] undefined!

Fixes: 1b11e893

 ("staging: rtl8723bs: replace private arc4 encryption with in-kernel one")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20210721153550.3624490-1-arnd@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>