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>

accel/tcg/cputlb.c

@@ -34,7 +34,7 @@
 #include "qemu/atomic.h"
 #include "qemu/atomic128.h"
 #include "translate-all.h"
-#include "trace-root.h"
+#include "trace/trace-root.h"
 #include "trace/mem.h"
 #ifdef CONFIG_PLUGIN
 #include "qemu/plugin-memory.h"

accel/tcg/trace.h

@@ -0,0 +1 @@
+#include "trace/trace-accel_tcg.h"

accel/tcg/user-exec.c

@@ -26,7 +26,7 @@
 #include "translate-all.h"
 #include "exec/helper-proto.h"
 #include "qemu/atomic128.h"
-#include "trace-root.h"
+#include "trace/trace-root.h"
 #include "trace/mem.h"
 
 #undef EAX