tools/hotplug: fix install rule

Remove dependency on 'all' because this causes make to recurse twice
into subdirs at the same time. Once with the 'all' target and once with
the 'install*' targets.

Spotted by Anthony PERARD.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Cc: Wei Liu <wei.liu2@citrix.com>