libvhost-user: Include poll.h instead of sys/poll.h

libvhost-user: Include poll.h instead of sys/poll.h

Musl libc complains about it's wrong usage.

In file included from ../subprojects/libvhost-user/libvhost-user.h:20,
                 from ../subprojects/libvhost-user/libvhost-user-glib.h:19,
                 from ../subprojects/libvhost-user/libvhost-user-glib.c:15:
/usr/include/sys/poll.h:1:2: error: #warning redirecting incorrect #include <sys/poll.h> to <poll.h> [-Werror=cpp]
    1 | #warning redirecting incorrect #include <sys/poll.h> to <poll.h>
      |  ^~~~~~~

Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20210118063808.12471-3-jiaxun.yang@flygoat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>