iwlwifi: Fix built-in compilation of iwlcore

This patch fixes problem in Makefile that prevented
built-in compilation of iwlcore

Commit that caused this problem: eadd3c4b ("iwlwifi: make Makefile
more concise")

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Yi Zhu <yi.zhu@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>