DiscIO: Make use of fmt where applicable

Once nice benefit of fmt is that we can use positional arguments
in localizable strings. This a feature which has been
requested for the Korean translation of strings like
"Errors were found in %zu blocks in the %s partition."
and which will no doubt be useful for other languages too.

Source/Core/DiscIO/CMakeLists.txt

@@ -63,6 +63,7 @@ PUBLIC
   zstd
 
 PRIVATE
+  fmt::fmt
   minizip
   pugixml
   ZLIB::ZLIB