hw/arm/iotkit.c: fix minor memory leak

hw/arm/iotkit.c: fix minor memory leak

Coverity (CID1390573) spots that we forgot to free the
gpioname strings in a loop in the iotkit realize function.
Correct the error.

This isn't a significant leak, because this function
only ever runs once.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Peter Xu <peterx@redhat.com>
Message-id: 20180427110137.19304-1-peter.maydell@linaro.org