hostapd has supported 802.11ac for quite a while now (and it has been
used in many OpenWrt/LEDE releases) so it should be safe to enable
802.11ac support in the build. This does not automatically turn it on
at runtime, but just makes it available when a config file tries to turn
it on (rather than spewing syntax errors about unrecognized directives).
Signed-off-by: Khem Raj <raj.khem@gmail.com>
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