drivers: flash: stm32wb: Fix build error

When building flash shell sample we get:

flash_stm32wbx.c:23:10: fatal error: shci.h: No such file or directory
   23 | #include "shci.h"

Fix this by adding ifdef protection around inclusion of shci.h.

Fixes #28036

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>