file_sys/directory: Make EntryType an enum class

This can trivially be an enum class rather than a regular enum, making
it more strongly typed.

src/core/file_sys/directory.h

@@ -15,7 +15,7 @@
 
 namespace FileSys {
 
-enum EntryType : u8 {
+enum class EntryType : u8 {
     Directory = 0,
     File = 1,
 };