declaration may not appear after executable statement in block

Hello, I was tryning to use MDK-ARM 5 I find some difficulties in its configuration.
This code is basically used to turn on the led that is connected to the pin 12 of the port D on the STM32F407VG. The problem is when I try to bild it, on error is appeared that says :

main.c(20): error: #268: declaration may not appear after executable statement in block GPIO_InitTypeDef GPIO_InitStructure;

#include "stm32f4xx.h"                  // Device header
#include "stm32f4xx_rcc.h"              // Keil::Device:StdPeriph Drivers:RCC
#include "stm32f4xx_gpio.h"             // Keil::Device:StdPeriph Drivers:GPIO

void GPIO(void);

int main(){

        GPIO();

        while(1){
                GPIO_SetBits(GPIOD, GPIO_Pin_12);
        }
}


void GPIO(){
        RCC_AHB1PeriphClockCmd(RCC_AHB1Periph_GPIOD, ENABLE);

        GPIO_InitTypeDef GPIO_InitStructure;

        GPIO_InitStructure.GPIO_Mode = GPIO_Mode_OUT;
        GPIO_InitStructure.GPIO_Pin = GPIO_Pin_12;
        GPIO_InitStructure.GPIO_OType = GPIO_OType_OD;
        GPIO_InitStructure.GPIO_PuPd = GPIO_PuPd_NOPULL;
        GPIO_InitStructure.GPIO_Speed = GPIO_Speed_2MHz;
        GPIO_Init(GPIOD, &GPIO_InitStructure);
}

Can you tell me guys what could be the problem with that. Thank you.