The include file lays the groundwork, it is not where the variable is defined. The error you posted is most definitely from the linker.
Find where you put the Repository and fish it out (alternatively download and unpack CubeF4 package)
STM32Cube_FW_F4_V1.24.2\Projects\STM32446E-Nucleo\Examples\GPIO\GPIO_IOToggle\Src\system_stm32f4xx.c
...
/** @addtogroup STM32F4xx_System_Private_Variables
* @{
*/
/* This variable is updated in three ways:
1) by calling CMSIS function SystemCoreClockUpdate()
2) by calling HAL API function HAL_RCC_GetHCLKFreq()
3) each time HAL_RCC_ClockConfig() is called to configure the system clock frequency
Note: If you use this function to configure the system clock; then there
is no need to call the 2 first functions listed above, since SystemCoreClock
variable is updated automatically.
*/
uint32_t SystemCoreClock = 16000000;
const uint8_t AHBPrescTable[16] = {0, 0, 0, 0, 0, 0, 0, 0, 1, 2, 3, 4, 6, 7, 8, 9};
const uint8_t APBPrescTable[8] = {0, 0, 0, 0, 1, 2, 3, 4};
...
