zephyr: rename 'led_init()' to 'io_led_init()'

This fixes below warning when building with 'MCUBOOT_INDICATION_LED'
enabled:

  mcuboot/boot/zephyr/main.c:410:5:
    warning: implicit declaration of function 'led_init';
             did you mean 'io_led_init'? [-Wimplicit-function-declaration]
      410 |     led_init();
          |     ^~~~~~~~
          |     io_led_init

Fixes: 433b848 ("zephyr: Move IO functions out of main to separate file")
Signed-off-by: Piotr Dymacz <[email protected]>

boot/zephyr/main.c

@@ -407,7 +407,7 @@ int main(void)
 
 #ifdef CONFIG_MCUBOOT_INDICATION_LED
     /* LED init */
-    led_init();
+    io_led_init();
 #endif
 
     os_heap_init();