ENOSYS-stubs for timer_* functions in libc

Hello,

OpenBSD 7.9 and -CURRENT have timer_* stubs functions
only ever returning ENOSYS.
See: lib/libc/sys/timer_*.c

Thankfully they are undeclared in the public headers, but clang(1)
doesn't defaults to -Werror=implicit-function-declaration and having
stubs means the linking step doesn't fails.

So broken compilation just continues on, and detecting lack
of support of the standard timer_* functions becomes a bit harder
than it could be.

Best regards