wireless: Skip directory when generating certificates

Commit 715a1233 ("wireless: don't write C files on failures") drops
the `test -f $$f` check. The list of targets contains the
CONFIG_CFG80211_EXTRA_REGDB_KEYDIR directory itself, and this check used
to filter it out. After the check was removed, the extra keydir option
no longer works, failing with the following message:

od: 'standard input': read error: Is a directory

This commit restores the check to make extra keydir work again.

Fixes: 715a1233 ("wireless: don't write C files on failures")
Signed-off-by: Maxim Mikityanskiy <maxtram95@gmail.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/wireless/Makefile

@@ -39,6 +39,7 @@ $(obj)/extra-certs.c: $(CONFIG_CFG80211_EXTRA_REGDB_KEYDIR:"%"=%) \
 	@(set -e; \
 	  allf=""; \
 	  for f in $^ ; do \
+	      test -f $$f || continue;\
 	      # similar to hexdump -v -e '1/1 "0x%.2x," "\n"' \
 	      thisf=$$(od -An -v -tx1 < $$f | \
 	                   sed -e 's/ /\n/g' | \