Fix "flexible array member X not at end of struct" compiling with GCC-11

[maurimo]

Fix a problem compiling with GCC-11 (present when I compiled latest Torch which includes gloo). The problem was also reported on Debian's ML, see for full context: https://www.mail-archive.com/[email protected]/msg1875386.html

This problem should not be occurring in the first place and it's probably a bug of GCC, but luckily the workaround is quite simple too.