When compiling with CONFIG_USE_RFKILL=y, the build fails and mentions that
dependency on kmod-rfkill is missing, which is correct [1]. Add this
dependency to the Makefile.

Depend on +USE_RFKILL and not PACKAGE_kmod-rfkill, because it forces
selection of kmod-rfkill package. Other combinations in DEPENDS like
USE_RFKILL:kmod-rfkill or (+)PACKAGE_kmod-rfkill:kmod-rfkill do not force
selection of kmod-rfkill package.

The kmod-rfkill package itself depends on USE_RFKILL, so with +USE_RFKILL
in kmod-cfg80211 package it is not possible to select wrong combination of
packages.

[1] https://linux-wireless.vger.kernel.narkive.com/m8JY9Iks/cfg80211-depends-on-rfkill-or-not



Signed-off-by: Oldřich Jedlička <oldium.pro@gmail.com>