samples: boards: st: build only this sample for stm32 boards

Makes the sample build only in automatic test CI
Running on the nucleo_f411re board requires hsi enable
Running on the nucleo_wba55cg can still output the console
on the usart1 Tx even if the usart1Rx is used by the MCO output

Signed-off-by: Francois Ramu <francois.ramu@st.com>