cfg80211: don't print log output for building shipped-certs

Building an allmodconfig kernel with 'make -s' now prints a single line:

  GEN     net/wireless/shipped-certs.c

Using '$(kecho)' here will skip the output with 'make -s' but
otherwise keeps printing it, which is consistent with how we
handle all the other output.

Fixes: 90a53e44 ("cfg80211: implement regdb signature checking")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>