trace: switch position of headers to what Meson requires

Meson doesn't enjoy the same flexibility we have with Make in choosing
the include path.  In particular the tracing headers are using
$(build_root)/$(<D).

In order to keep the include directives unchanged,
the simplest solution is to generate headers with patterns like
"trace/trace-audio.h" and place forwarding headers in the source tree
such that for example "audio/trace.h" includes "trace/trace-audio.h".

This patch is too ugly to be applied to the Makefiles now.  It's only
a way to separate the changes to the tracing header files from the
Meson rewrite of the tracing logic.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

job-qmp.c

@@ -27,7 +27,7 @@
 #include "qemu/job.h"
 #include "qapi/qapi-commands-job.h"
 #include "qapi/error.h"
-#include "trace-root.h"
+#include "trace/trace-root.h"
 
 /* Get a job using its ID and acquire its AioContext */
 static Job *find_job(const char *id, AioContext **aio_context, Error **errp)