CMake: Use find_package and imported targets for Iconv

Fixes us forgetting to add its include directories, which could result in linking to a dylib from MacPorts while using the system's header, and failing to link because they use different function names

Source/Core/Common/CMakeLists.txt

@@ -156,7 +156,7 @@ PUBLIC
 PRIVATE
   ${CURL_LIBRARIES}
   FatFs
-  ${ICONV_LIBRARIES}
+  Iconv::Iconv
   ${spng_target}
   ${VTUNE_LIBRARIES}
 )