glibc ipc64_perm __key compatibility.

glibc gives the `struct ipc64_perm` fields `key` and `seq` double-underscore
names. strace refers to the fields by those names, and there's no obvious
reason not to go along with this.

Bug: N/A
Test: built strace 4.15 with a hacked NDK
Change-Id: I8b2b0f75363349d99edaecda50fe897ee0fa0c65