[DFSAN] Silence parameter 'va_labels' set but not used warning (NFC)

/data/llvm-project/compiler-rt/lib/dfsan/dfsan_custom.cpp:2546:37: error: parameter 'va_labels' set but not used [-Werror,-Wunused-but-set-parameter]
                       dfsan_label *va_labels, dfsan_label *ret_label,
                                    ^
1 error generated.

compiler-rt/lib/dfsan/dfsan_custom.cpp

@@ -2754,6 +2754,7 @@ static int scan_buffer(char *str, size_t size, const char *fmt,
     formatter.str_off += retval;
   }
 
+  (void)va_labels; // Silence unused-but-set-parameter warning
   *ret_label = 0;
   if (ret_origin)
     *ret_origin = 0;