yahoo: Add yahoojp.c to EXTRA_DIST

yahoojp.c is in POTFILES.in, but isn't actually use for the build.
This causes `make distcheck` to fail as the file isn't "distributed".
This patch adds yahoojp.c to EXTRA_DIST so it will be distributed
and successfully checked when generating the translation files.

Really this file could be removed, but the 2.x.y branch has all of
the Yahoo prpl removed and that will be merged here soon enough.