Core/CMakeLists: Privately link in fmt

While we do have this library as part of the public linkage interface in
the common library target, which will be used in the future for the
logging macros, we should still be explicit that we're using this
library. Therefore, we privately link it in to be explicit about it.

Source/Core/Core/CMakeLists.txt

@@ -557,6 +557,7 @@ PUBLIC
   videovulkan
 
 PRIVATE
+  fmt::fmt
   ${LZO}
   ZLIB::ZLIB
 )