Add the easy_codec plugin to the source distribution.

It doesn't build on Linux but cmake looks for it (won't build without it) and
Windows users might want it.

Change-Id: I978f0de0a2895a82f4f3b8c1e9e0ecec6a93e6f4
Reviewed-on: https://code.wireshark.org/review/3325
Reviewed-by: Jeff Morriss <jeff.morriss.ws@gmail.com>