Make the sandbox code allow the writev() syscall.

Tor doesn't use it directly, but the glibc backtrace-to-fd code does

src/common/sandbox.c

@@ -119,6 +119,7 @@ static int filter_nopar_gen[] = {
     SCMP_SYS(stat),
     SCMP_SYS(uname),
     SCMP_SYS(write),
+    SCMP_SYS(writev),
     SCMP_SYS(exit_group),
     SCMP_SYS(exit),