brcmfmac: include also core.h header in cfg80211.h

brcmfmac: include also core.h header in cfg80211.h

This header provides two inline functions using struct brcmf_if so we
need core.h to avoid:

drivers/net/wireless/broadcom/brcm80211/brcmfmac/cfg80211.h: In function ‘ndev_to_prof’:
drivers/net/wireless/broadcom/brcm80211/brcmfmac/cfg80211.h:368:13: error: dereferencing pointer to incomplete type
  return &ifp->vif->profile;
             ^
drivers/net/wireless/broadcom/brcm80211/brcmfmac/cfg80211.h: In function ‘ndev_to_vif’:
drivers/net/wireless/broadcom/brcm80211/brcmfmac/cfg80211.h:374:12: error: dereferencing pointer to incomplete type
  return ifp->vif;
            ^

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>