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 <npes87184@gmail.com>
Signed-off-by: Romain Vimont <rom@rom1v.com>

cross_win32.txt

@@ -22,4 +22,4 @@ ffmpeg_avutil = 'avutil-56'
 prebuilt_ffmpeg = 'ffmpeg-win32-4.3.1'
 prebuilt_sdl2 = 'SDL2-2.0.22/i686-w64-mingw32'
 prebuilt_libusb_root = 'libusb-1.0.26'
-prebuilt_libusb = prebuilt_libusb_root + '/MinGW-Win32'
+prebuilt_libusb = 'libusb-1.0.26/MinGW-Win32'

cross_win64.txt

@@ -22,4 +22,4 @@ ffmpeg_avutil = 'avutil-57'
 prebuilt_ffmpeg = 'ffmpeg-win64-5.0.1'
 prebuilt_sdl2 = 'SDL2-2.0.22/x86_64-w64-mingw32'
 prebuilt_libusb_root = 'libusb-1.0.26'
-prebuilt_libusb = prebuilt_libusb_root + '/MinGW-x64'
+prebuilt_libusb = 'libusb-1.0.26/MinGW-x64'