tests/periph_uart: fix for !stdio_uart
tests/periph_uart: fix for !stdio_uart

Only include `stdio_uart.h` if stdio over UART is used.
This makes it possible to use the default stdio UART0
for the test.

This is needed if e.g. the default stdio of a board is USB CDC ACM.

authmillenon/RIOT,OTAkeys/RIOT,jasonatran/RIOT,kYc0o/RIOT,kYc0o/RIOT,miri64/RIOT,OTAkeys/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,OlegHahm/RIOT,RIOT-OS/RIOT,authmillenon/RIOT,basilfx/RIOT,kYc0o/RIOT,ant9000/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,authmillenon/RIOT,ant9000/RIOT,OTAkeys/RIOT,OlegHahm/RIOT,ant9000/RIOT,RIOT-OS/RIOT,basilfx/RIOT,authmillenon/RIOT,authmillenon/RIOT,kaspar030/RIOT,miri64/RIOT,OlegHahm/RIOT,miri64/RIOT,kaspar030/RIOT,OTAkeys/RIOT,ant9000/RIOT,OlegHahm/RIOT,basilfx/RIOT,OTAkeys/RIOT,basilfx/RIOT,OlegHahm/RIOT,miri64/RIOT,jasonatran/RIOT,kaspar030/RIOT,kaspar030/RIOT,kYc0o/RIOT,authmillenon/RIOT,kaspar030/RIOT,miri64/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,basilfx/RIOT,ant9000/RIOT,jasonatran/RIOT