Autogenerated code in trace.h/trace.c and friends is specific to the
config-host.mak TRACE_BACKENDS setting and must be regenerated when
./configure --enable-trace-backend= changes settings.

This patch ensures that changes to TRACE_BACKENDS are detected.  For
example, the trace-root.h file is now updated after switching trace
backends:

  $ ./configure && make
  $ cp trace-root.h /tmp/old-trace-root.h
  $ ./configure --enable-trace-backend=simple && make
  $ diff -u /tmp/old-trace-root.h trace-root.h

Reported-by: Christophe Lyon <christophe.lyon@st.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-id: 20190129025343.4788-1-stefanha@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>