The "stamp file trick" used to group targets of a single multi-output rule
prevents the user from deleting one such target in order to force its
rebuild.  Doing so will not touch the stamp file, and therefore only
the dummy ":" command will be executed.

With this patch, ninjatool writes rules that force-rebuild the stamp
file if any of its outputs are missing.  Rebuilding the missing
target therefore causes the stamp file to be rebuilt too.

Reported-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>