keil报错Undefined symbol HAL_PWRxx_xx (referred from xxxx.o)
如下：
.\Template.axf: Error: L6218E: Undefined symbol HAL_PWREx_GetVoltageRange (referred from stm32l4xx_hal_rcc.o).
.\Template.axf: Error: L6218E: Undefined symbol HAL_PWR_DisableBkUpAccess (referred from stm32l4xx_hal_rcc_ex.o).
.\Template.axf: Error: L6218E: Undefined symbol HAL_PWR_EnableBkUpAccess (referred from stm32l4xx_hal_rcc_ex.o).
.\Template.axf: Error: L6218E: Undefined symbol HAL_PWREx_ControlVoltageScaling (referred from sys.o).
这种一般都是没有stm32l4xx_hal_pwr.c、stm32l4xx_hal_pwr_ex.c文件，添加源文件后编译正常。
