rtos: fix iar vector table type

Following IAR guide for RAM vector table, this is in the code snippet there:

```
extern void * __vector_RAM_table [];
void main(void)
```

Found in #12547

Author: 0xc0170

rtos/source/TARGET_CORTEX/TOOLCHAIN_IAR/mbed_boot_iar.c

@@ -28,7 +28,7 @@
 #pragma section="CSTACK"
 #pragma section="HEAP"
 
-extern void *__vector_table;
+extern void *__vector_table[];
 extern int  __low_level_init(void);
 extern void __iar_data_init3(void);
 extern __weak void __iar_init_core(void);