[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.