Makefile: Fix "make install" when "make all" needs work

Until recently, target install used to recurse into target directories
in its recipe: it ran make install in a for-loop.  Since target
install depends on target all, this trivially ensured we run the
sub-make install only after completing target all.

Commit 1338a4b7 "Makefile: Reuse all's recursion machinery for clean
and install" moved the target recursion to dependencies.  That's good
(the commit message explains why), but I forgot to add dependencies to
ensure make runs the sub-make install only after completing target
all.  Do that now.

Fixes: 1338a4b7
Reported-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reported-by: Guenter Roeck <linux@roeck-us.net>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20190712055935.23061-1-armbru@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Tested-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>