Supply a local copy of L-SMASH's importer.h in extras/lsmash/

This relieves the user of having to install this particular header
themselves with a different name, or from having to patch L-SMASH
to install it alongside lsmash.h.

The original move to liblsmash was intended to do away with having
to supply L-SMASH's source code internally, and this doesn't
change that.  It's simply easier to supply one compatibility
header than to force the user to know to copy it to the system
as lsmash_importer.h or to mess with L-SMASH's installation rules.