scripts/kernel-doc: fix fatal error caused by cfg80211.h

include/net/cfg80211.h uses __must_check in functions that
have kernel-doc notation.  This was confusing scripts/kernel-doc,
so have scripts/kernel-doc ignore "__must_check".

Error(include/net/cfg80211.h:2702): cannot understand prototype: 'struct cfg80211_bss * __must_check cfg80211_inform_bss(...)

Signed-off-by: Randy Dunlap <rdunlap@xenotime.net>
Cc:	Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>