sandbox-seccomp-filter: allow gettid

Some allocators (such as Scudo) use gettid while tracing allocations [1].
Allow gettid in preauth to prevent sshd from crashing with Scudo.

[1]: https://github.com/llvm/llvm-project/blob/llvmorg-13.0.0/compiler-rt/lib/gwp_asan/common.cpp#L46

sandbox-seccomp-filter.c

@@ -228,6 +228,9 @@ static const struct sock_filter preauth_insns[] = {
 #ifdef __NR_getrandom
 	SC_ALLOW(__NR_getrandom),
 #endif
+#ifdef __NR_gettid
+	SC_ALLOW(__NR_gettid),
+#endif
 #ifdef __NR_gettimeofday
 	SC_ALLOW(__NR_gettimeofday),
 #endif