skalibs: fix bashism in type sizes patch

Instead of using a Bash-only variable substitution feature in the patch
to determine type size at build time, invoke `sed` to make it portable.

Fixes:

  http://autobuild.buildroot.net/results/023d71356376bea13d16c43898a0347e41cd19b9/



Signed-off-by: Eric Le Bihan <eric.le.bihan.dev@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>