MinGW compilation warnings in libiberty's waitpid.c

[Joel Brobecker]

> Thanks.  Is the environ thing also fixed?
> 
> Joel/Pedro, how should I go about making sure these changes are in the
> GDB copy of libiberty?

Normally, I'd expect someone pushing to GCC's libibert to also
update our repo accordingly. However, it's easy to forget so,
if you notice a change that was not propagated to us, we just
cherry-pick those changes so as to make sure our copy is up
to date with GCC's. We also see the occasional "resync libiberty"
commits which act as a failsafe, but I don't think we should wait
for one of those.

Should those ones be pushed to the gdb-8.0-branch as well?

-- 
Joel