Meta: Add allow-list for icon size detection in CMake

The only icons we are currently warning about are designed
and rendered as small icons intentionally, as their only use
is in desktop applets, and thus are exempt to this rule.

This reduces build spam back down to a minimum.
I should have just done this in the first place, back in SerenityOS#4729

Meta/CMake/utils.cmake

@@ -86,7 +86,13 @@ function(serenity_app target_name)
  if (EXISTS "${medium_icon}")
  embed_resource("${target_name}" serenity_icon_m "${medium_icon}")
  else()
- message(WARNING "Missing medium app icon: ${medium_icon}")
+ # These icons are designed small only for use in applets, and thus are exempt.
+ list(APPEND allowed_missing_medium_icons "audio-volume-high")
+ list(APPEND allowed_missing_medium_icons "edit-copy")
+
+ if (NOT ${SERENITY_APP_ICON} IN_LIST allowed_missing_medium_icons)
+ message(WARNING "Missing medium app icon: ${medium_icon}")
+ endif()
  endif()
 endfunction()