The -k flag has been removed from Makefile.common

Change-Id: Ib7399c4b5457c5810df3ce2160a56813785b11ec
Reviewed-on: https://code.wireshark.org/review/3498
Petri-Dish: Stig Bjørlykke <stig@bjorlykke.org>
Reviewed-by: Stig Bjørlykke <stig@bjorlykke.org>
Tested-by: Stig Bjørlykke <stig@bjorlykke.org>