Historically, including some headers ended up including some other headers automatically, such that the author might not realize what headers the file depends on, because it compiles without them explicitly included. glibc (and maybe gcc and xlib?) have been cleaning this up where possible, slowly over time. So a file that had "correct enough" includes for an older glibc might now be caught not including a header it needed all along.
Or maybe it's a similar situation but actually related to "feature test macros" (see https://lwn.net/Articles/590381/)