lib/mkmf.rb: sort list of object files in generated Makefile

lib/mkmf.rb: sort list of object files in generated Makefile

Without sorting the list explicitely, its order is indeterministic,
because readdir() is also not deterministic.
When the list of object files varies between builds, they are linked
in a different order, which results in an unreproducible build.

Signed-off-by: Antonio Terceiro <terceiro@debian.org>
Signed-off-by: Reiner Herrmann <reiner@reiner-h.de>
Gbp-Pq: Name 0002-lib-mkmf.rb-sort-list-of-object-files-in-generated-M.patch