Error in programming STM32F4 Discovery with IAR?
The linker is looking for the function SYSCFG_EXTILineConfig() but can't find it, probably because the source file is missing from your workspace.  The missing file is probably named something like stm32f4xx_syscfg.c

BTW, the "definition of SYSCFG_EXTILineConfig in .h file" is a declaration (prototype), not a definition.