scripts: size_report: fix bug where key is used as a dict
scripts: size_report: fix bug where key is used as a dict

When printing the unassigned values the 'sym' variable is
used as a dict from which we try to get the 'name' value.
However, 'symbols['unassigned']' gives a list of keys, so
we get an 'TypeError' when trying to access ['name'] of
a string (the key).

Fix this issue by iterating over the values from
the 'symbols['unassigned']' dict instead.

Signed-off-by: Håkon Øye Amundsen <4238ca2ac515887d8a25d5e30932b12519303bf7@nordicsemi.no>

galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,finikorg/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,Vudentz/zephyr,Vudentz/zephyr,finikorg/zephyr,Vudentz/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,nashif/zephyr,galak/zephyr,nashif/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,nashif/zephyr,nashif/zephyr,finikorg/zephyr,galak/zephyr