instance.c doesn't need to include dlfcn

This should have been removed a long time ago..

Change-Id: I6ce74e4cbe2a0bb2ba11d8b70ab463e459181c86
Reviewed-on: http://review.couchbase.org/38536
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Brett Lawson <brett19@gmail.com>