package/rtl8723bu: fix build failure due to missing Linux options

This driver requires:
CONFIG_NET
CONFIG_WIRELESS
CONFIG_CFG80211
CONFIG_USB_SUPPORT
CONFIG_USB
to build so let's add them to RTL8723BU_LINUX_CONFIG_FIXUPS.

Fixes:
http://autobuild.buildroot.net/results/7f3637760ad88d314e6812865f3667d1de091957/



Signed-off-by: Giulio Benetti <giulio.benetti@benettiengineering.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>