packagegroup-sdk-target: drop libltdl-dev dependency

* it was removed from libtool recipe in oe-core commit:
  commit 5e40bcd93d9ea64f1a6d75e62aabda3dcb33fc01
  Author: Robert Yang <liezhi.yang@windriver.com>
  Date:   Fri Apr 10 02:30:02 2015 -0700
  Subject: libtool: remove libltdl-dev libltdl-dbg libltdl-staticdev from PACKAGES

    There should be only one dev and dbg package

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>