samples: drivers: flash: Add missing fixture requirement
samples: drivers: flash: Add missing fixture requirement

This sample can only be tested if the DUT has external memory
connected. This commit aligns the yaml description, so the test
is not picked up on setups without required fixture.

Signed-off-by: Maciej Perkowski <cb34e965b6a5a3425eafdbd0feb7e469ae1417ea@nordicsemi.no>

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