libglnx.m4: Include stdio.h for renameat2

glibc added it upstream: https://sourceware.org/git/?p=glibc.git;a=commit;h=d6da5cb6a8e0e8a9ce92b7d951a254cf325248d7

But we need the right header. Ref: https://github.com/flatpak/flatpak/issues/1890