ext/hal: stm32cube: fix exti declaration in STM32L4

LL_EXTI_LINE_18 and LL_EXTI_LINE_20 are declared in stm32l4xx_hal_pcd.h
and in stm32l4xx_ll_exti.h which generates warnings. Set #ifndef
in stm32l4xx_ll_exti.h around declarations.
STBugtracker: 55275

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>
2 files changed