add a missing extern to a global declaration of a status register

Author: Rohit Grover

hal/targets/hal/TARGET_Freescale/TARGET_KSDK2_MCUS/TARGET_K64F/storage_driver.c

@@ -124,7 +124,7 @@
 /*@}*/
 
 /*! @brief Access to FTFx->FCCOB */
-volatile uint32_t *const kFCCOBx;
+extern volatile uint32_t *const kFCCOBx;
 
 static flash_config_t privateDeviceConfig;
 #endif /* #ifdef USING_KSDK2 */