Makefile: Keep trace-events-subdirs ordered

Makefile: Keep trace-events-subdirs ordered

Adding the same directory multiple times to trace-events-subdirs
might trigger build failures, in particular when using the LTTng
Userspace Tracer library as backend.

For example when using two times the hw/core/ directory:

    $ ./configure --enable-trace-backends=ust && make
   [...]
     CC      trace-ust-all.o
   In file included from trace-ust-all.h:13,
                    from trace-ust-all.c:13:
   trace-ust-all.h:35151:1: error: redefinition of ‘__tracepoint_cb_qemu___loader_write_rom’
   35151 | TRACEPOINT_EVENT(
         | ^~~~~~~~~~~~~~~~
   trace-ust-all.h:31791:1: note: previous definition of ‘__tracepoint_cb_qemu___loader_write_rom’ was here
   31791 | TRACEPOINT_EVENT(
         | ^~~~~~~~~~~~~~~~

To ease review and reduce likelihood of merge failures (see [*]),
keep trace-events-subdirs ordered when possible, following eb7ccb3c0.

[*] https://www.mail-archive.com/qemu-devel@nongnu.org/msg671007.html
    Duplicate trace-events-subdirs entries generates duplicated
    symbols when using the LTTng Userspace Tracer library.

Reviewed-by: Damien Hedde <damien.hedde@greensocs.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-id: 20200116114339.30670-1-philmd@redhat.com
Message-Id: <20200116114339.30670-1-philmd@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>