trace: ensure $(tracetool-y) is defined in top level makefile

The build rules for trace files have a dependancy on $(tracetool-y).
This variable populated in the trace/Makefile.objs file and thus its
definition gets pulled into the top level makefile. This happens too
late in the process though, so by the time $(tracetool-y) is defined,
make has already evaluated $(tracetool-y) in the dependancies and
found it to be empty. The result is that when the tracetool source
is changed, the generated files are not rebuilt. The solution is to
define the variable in the top level makefile too

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Tested-by: Eric Blake <eblake@redhat.com>
Message-id: 20170315123421.28815-1-berrange@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>