soc: arm: stm32lx: fix using MSI as wake-up clock source

Following the migration of the clock source configuration in DTS (commit
2691541a), HSI is always used as wake-up source on STM32LX. It is
reconfigured as MSI just after, but it slightly increase the wake-up
time and power consumption.

It happens as the file defining STM32_SYSCLK_SRC_MSI is not included.
Fix that.

Fixes #38807

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>