package/autossh: do not look for the host's ssh command

autossh looks for 'ssh' in the PATH, which may not exist on a build
machine.

Fixes a private autobuild failure (soon to go public).

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Peter Korsgaard <jacmet@uclibc.org>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>