Add some files under asn1 to the distribution.

Include CMakeLists.txt files and the gnm subdirectory, along with the
top-level Makefile.inc and Makefile.preinc files.  Don't explicitly
include Custom.make, as automake does that automatically given that it's
included by asn1/Makefile.am.

Add some files to EXTRA_DIST lists.

Move some .asn files to EXTRA_DIST; they don't need to be in SRC_FILES,
as SRC_FILES always includes EXTRA_DIST, and they *do* need to be in
EXTRA_DIST so that they're in the distribution.

Change-Id: Id91df577260fa57028d40fe098be1d79c59398e6
Reviewed-on: https://code.wireshark.org/review/3273
Reviewed-by: Guy Harris <guy@alum.mit.edu>