package/s390-tools: fix zkey build

Linking of libekmfweb fails when zkey-ekmfweb.so build is enabled.

Fixes:
- http://autobuild.buildroot.net/results/d41bca3be35d1a48f962be03920f2b81c3e9bb9f



Signed-off-by: Alexander Egorenkov <egorenar@linux.ibm.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>