Avoid string concatenation in crossfiles

This feature is not supported on older meson versions:

    ERROR: Malformed value in cross file variable prebuilt_libusb.

Refs <https://github.com/mesonbuild/meson/issues/3878>
PR #3546 <https://github.com/Genymobile/scrcpy/pull/3546>

Signed-off-by: Yu-Chen Lin <[email protected]>
Signed-off-by: Romain Vimont <[email protected]>