sysutils/nut-devel: Only install nut-usb.conf with USB option

${LOCALBASE}/etc/devd/nut-usb.conf was installed unconditionally. This is
incorrect. It should only be installed when the USB option is selected.

MFH: 2023Q2
(cherry picked from commit 3d2be436ee8b8269c3e2a63fe42616705b2270dd)