simpletrace: suppress a warning from unused variable

Avoid this warning:
  CC    simpletrace.o
/src/qemu/simpletrace.c: In function 'writeout_thread':
/src/qemu/simpletrace.c:122:12: error: variable 'unused' set but not used [-Werror=unused-but-set-variable]
by adding GCC attribute unused to the variable.
Signed-off-by: NBlue Swirl <blauwirbel@gmail.com>

simpletrace.c

@@ -119,7 +119,7 @@ static void *writeout_thread(void *opaque)
     TraceRecord record;
     unsigned int writeout_idx = 0;
     unsigned int num_available, idx;
-    size_t unused;
+    size_t unused __attribute__ ((unused));
 
     for (;;) {
         wait_for_trace_records_available();