Trace: Avoid redefine of UNUSED() macro

The xt-clang RI-2022.10 compiler headers define UNUSED.
With this patch the definition is done only if the macro is
undefined. This fixes error:

  In file included sof/tools/tplg_parser/pga.c:17:
  In file included sof/tools/tplg_parser/
  ../../src/include/sof/lib/uuid.h:11:
  In file included sof/tools/tplg_parser/
  ../../src/include/sof/common.h:105:
  /home/singalsu/work/current/sof/sof/tools/tplg_parser/
  ../../src/include/sof/trace/preproc.h:162:9: error:
  'UNUSED' macro redefined [-Werror,-Wmacro-redefined]
  #define UNUSED(arg1, ...) do { META_RECURSE(
  xtensa/XtDevTools/install/tools/RI-2022.10-linux/XtensaTools/
  xtensa-elf/include/xtensa/xtensa-types.h:60:9:
  note: previous definition is here
  #define UNUSED(x)       ((void)(x))

Signed-off-by: Seppo Ingalsuo <[email protected]>

src/include/sof/trace/preproc.h

@@ -158,8 +158,10 @@
 		return_t META_CONCAT(prefix, postfix) (args)
 
 /* counteract compiler warning about unused variables */
+#ifndef UNUSED
 #define UNUSED(arg1, ...) do { META_RECURSE( \
 	META_MAP_AGGREGATE(1, _META_VOID2, _META_VOID(arg1), __VA_ARGS__)); \
 	} while (0)
+#endif
 
 #endif /* __SOF_TRACE_PREPROC_H__ */