openssh: change libexecdir for sftp-server

By default, bitbake.conf sets libexecdir to /usr/lib/openssh,
but dropbear expects to find sftp-server from /usr/libexec

Change-Id: I19b2051013a9c67e7c09b0815a926334faf99451
Reviewed-by: Kalle Viironen <kalle.viironen@digia.com>

recipes/openssh/openssh_6.1p1.bbappend

+export libexecdir="/usr/libexec"
+