security/opensc: Fix sample config install

Error: Missing: %%ETCDIR%%.conf.sample
[...]
Installing opensc-0.19.0...
cp: /usr/local/etc/opensc.conf.sample: No such file or directory

The build uses USES=autoreconf and patching etc/Makefile.in is a
nop. Patch etc/Makefile.am instead.

Broken in r500489.