libc: remove unused errno.h include

errno.h was added in 44cf1e5eb470, which has been reverted.

Fixes: d09a3bf72c0b ("fflush: correct buffer handling in __sflush")
Sponsored by: The FreeBSD Foundation

(cherry picked from commit 418f026bd5a5084c1c4e2e91ad38051f6caa928c)

Approved by: so