Fix builds --without-usb

[jimklimov]

Closes: #2262
Follow-up from #2063 et al

CC @EchterAgo

• Configured --without-usb (now gets built):

:; ldd drivers/apc_modbus
        linux-vdso.so.1 (0x00007fff21ffa000)
        libsystemd.so.0 => /lib/x86_64-linux-gnu/libsystemd.so.0 (0x00007ff2ecd20000)
        libmodbus.so.5 => /lib/x86_64-linux-gnu/libmodbus.so.5 (0x00007ff2ecd12000)
        libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007ff2ecaea000)
        liblzma.so.5 => /lib/x86_64-linux-gnu/liblzma.so.5 (0x00007ff2ecabf000)
        libzstd.so.1 => /lib/x86_64-linux-gnu/libzstd.so.1 (0x00007ff2ec9f0000)
        liblz4.so.1 => /lib/x86_64-linux-gnu/liblz4.so.1 (0x00007ff2ec9ce000)
        libcap.so.2 => /lib/x86_64-linux-gnu/libcap.so.2 (0x00007ff2ec9c3000)
        libgcrypt.so.20 => /lib/x86_64-linux-gnu/libgcrypt.so.20 (0x00007ff2ec885000)
        /lib64/ld-linux-x86-64.so.2 (0x00007ff2ece18000)
        libgpg-error.so.0 => /lib/x86_64-linux-gnu/libgpg-error.so.0 (0x00007ff2ec85f000)

• Configured by default (detecting libusb and using it):

:; ldd drivers/apc_modbus
        linux-vdso.so.1 (0x00007ffd977d7000)
        libsystemd.so.0 => /lib/x86_64-linux-gnu/libsystemd.so.0 (0x00007f357e5c8000)
        libmodbus.so.5 => /lib/x86_64-linux-gnu/libmodbus.so.5 (0x00007f357e5ba000)
        libusb-1.0.so.0 => /lib/x86_64-linux-gnu/libusb-1.0.so.0 (0x00007f357e59c000)
        libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f357e374000)
        liblzma.so.5 => /lib/x86_64-linux-gnu/liblzma.so.5 (0x00007f357e349000)
        libzstd.so.1 => /lib/x86_64-linux-gnu/libzstd.so.1 (0x00007f357e278000)
        liblz4.so.1 => /lib/x86_64-linux-gnu/liblz4.so.1 (0x00007f357e258000)
        libcap.so.2 => /lib/x86_64-linux-gnu/libcap.so.2 (0x00007f357e24d000)
        libgcrypt.so.20 => /lib/x86_64-linux-gnu/libgcrypt.so.20 (0x00007f357e10f000)
        /lib64/ld-linux-x86-64.so.2 (0x00007f357e6c5000)
        libudev.so.1 => /lib/x86_64-linux-gnu/libudev.so.1 (0x00007f357e0e5000)
        libgpg-error.so.0 => /lib/x86_64-linux-gnu/libgpg-error.so.0 (0x00007f357e0bd000)

[EchterAgo]

Looks good to me.