scripts: size_report: add tree node for hidden symbols
scripts: size_report: add tree node for hidden symbols

The list of symbols only contain visible symbols that can be
obtained through ELF file. There are code and data where
the toolchain never emits symbols for them and thus are not
visible in the list. So add a "(hidden)" node to the tree
to show they are there.

Signed-off-by: Daniel Leung <d94e04d205b5962d7873a139ef298b03c1717f72@intel.com>

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