Partially fix 'make dist'
This incorporates an upstream fix to add jdmrg565.c to the tarball created
by 'make dist', as well as a fix to add the new jcmaster.h file to same.  There
are still some mozjpeg-specific files that aren't added when doing 'make dist'.
I'll let someone else worry about those.  This patch mainly ensures that any
files that might be eventually adopted upstream are included.