CI: travis-ci: use proper linking flags for SLZ build

previously SSL_INC and SSL_INC were set for all builds, and SLZ lib
was linked because of those flags. After we switched SLZ build to
stock openssl lib, SSL_INC, SSL_LIB are not set anymore.

Good time to set SLZ_INC, SLZ_LIB for such builds
1 file changed