ecm_add_app_icon() failed to generate ico file from png probably bc missing libpng

[Wang Gary]

Hello,

I'm trying to compile kdeconnect-kde under Windows via
KDE Craft, and it failed at `Generating indicator_SRCS.ico`.
At first I try to comment out the `ecm_add_app_icon(indicator_SRCS ICONS ...)`
lines in indicator/CMakeLists.txt and it compiles without any
other issues. Then I try to run the command manually which
do generate indicator_SRCS.ico, and then found out it's missing
libpng16.dll. Then I manually installed libpng via craft and continue
my kdeconnect-kde build, then it can build successfully without
issue.

I'm not sure if the kdeconnect-kde missed to add libpng as a
build dep in the blueprint or I did miss something in my craft
setup, I did found a similar issue in this mailing list but I'm not
sure about if it's the same case:
https://mail.kde.org/pipermail/kde-windows/2019-December/011042.html

Thanks
Gary