CCBC-513, CCBC-514: Fix helper header paths

This makes autotools install the 'helper' headers (bsdio-inl.c and such)
in their proper paths (i.e. include/libcouchbase/plugins/io/..) rather
than at the top-level include/libcouchbase.

Change-Id: I805c592c7e3a0fcad73e0e14fbc82795b2e0940c
Reviewed-on: http://review.couchbase.org/41450
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>