sd-boot: cast away reboot_into_firmware() return type

As mentioned by Lennart, in a commit where I was adding similar piece of
code:

  maybe cast this call to void, to tell static analyzers that we are
  ignoring the return value on purpose, not by accident

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>

src/boot/efi/boot.c

@@ -825,7 +825,7 @@ static bool menu_run(
                 if (firmware_setup) {
                         firmware_setup = false;
                         if (IN_SET(key, KEYPRESS(0, 0, '\r'), KEYPRESS(0, 0, '\n')))
-                                reboot_into_firmware();
+                                (void) reboot_into_firmware();
                         continue;
                 }