util/base64.c: Clean includes

Remove unnecessary include of config-host.h.
(This was missed by the clean-includes script because of the
incorrect use of <> for a QEMU header.)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-id: 1456237112-32662-5-git-send-email-peter.maydell@linaro.org