mpers.awk: relax union member name absence check
mpers.awk: relax union member name absence check

This fixes mpersing of unions containing nameless members,
e.g. struct btrfs_ioctl_vol_args_v2.

* mpers.awk (what_is): Print names of union_type members as is.

cuviper/strace,cuviper/strace,cuviper/strace,cuviper/strace,cuviper/strace