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 b62c381591,
when epoll_wait changed from being in syscalls.list to always having a C
wrapper.

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

ChangeLog

@@ -1,3 +1,7 @@
+2017-11-15  Luke Shumaker  <lukeshu@parabola.nu>
+
+	* sysdeps/unix/sysv/linux/epoll_wait.c: Include <sysdep-cancel.h>.
+
 2017-11-15  Mike FABIAN  <mfabian@redhat.com>
 
 	* localedata/locales/ka_GE (LC_MESSAGES): Add “X” back to yesexpr,

sysdeps/unix/sysv/linux/epoll_wait.c

@@ -21,6 +21,8 @@
 #include <sys/types.h>
 #include <sys/epoll.h>
 
+#include <sysdep-cancel.h>
+
 int
 epoll_wait (int epfd, struct epoll_event *events, int maxevents, int timeout)
 {