linux: Include <sysdep-cancel.h> for epoll_wait

The epoll_wait wrapper uses the raw syscall if __NR_epoll_wait is defined,
and falls back to calling epoll_pwait(..., NULL) if it isn't defined.
However, it didn't include the appropriate headers for __NR_epoll_wait to
be defined, so it was *always* falling back to calling epoll_pwait!

This mistake was introduced in b62c3815,
when epoll_wait changed from being in syscalls.list to always having a C
wrapper.

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>