community/firefox: add mcookie to makedepends
Fix build on 64b package builders, currently failing with:
>>> firefox: profiling firefox
/usr/bin/xvfb-run: line 155: mcookie: not found
>>> ERROR: firefox: build failed
This seems to be installed during build in our Gitlab CI but not on the package builders. It is only needed by 64b architectures where profiling firefox is done.