tests: drivers: buildall: modem: exclude litex_vexriscv
tests: drivers: buildall: modem: exclude litex_vexriscv

Excluding litex_vexriscv as UART_LITEUART doesn't support interrupts.

Signed-off-by: Yong Cong Sin <d1951b00420cc51c2da597170bb83d89fa46ae3b@gmail.com>

galak/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,finikorg/zephyr,finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,finikorg/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr