Include doc/arm/isc-logo.pdf in source tarballs

The doc/arm/conf.py Sphinx configuration file specifies doc/arm/isc-logo.pdf as the logo to use in the PDF files produced. Since doc/arm/isc-logo.pdf is not currently included in source tarballs produced using "make dist", attempting to build documentation in PDF format using a source tarball results in the following error being raised:

Sphinx error:
logo file 'isc-logo.pdf' does not exist

Ensure doc/arm/isc-logo.pdf is included in source tarballs produced using "make dist", so that the BIND 9 ARM can be successfully built in PDF format using just the source tarball.

Closes #3032 (closed)