Put asn1 back as a distributed subdirectory.

If it's not there, "make distcheck" fails.  In addition, 1) "make
distclean" doesn't clean out the Makefile.in files under asn1 and 2) you
don't get to see the source from which the ASN.1 dissectors were built
if you have a source tarball.

Change-Id: Ied59d42950373476a535fcf9cfccece46de92599
Reviewed-on: https://code.wireshark.org/review/4013
Reviewed-by: Guy Harris <guy@alum.mit.edu>