loaders: Make GetFileType() a const qualified member function

No implementations actually modify instance state (and it would be
questionable to do that in the first place given the name), so we can
make this a const member function.

src/core/loader/nsp.h

@@ -31,7 +31,7 @@ public:
      */
     static FileType IdentifyType(const FileSys::VirtualFile& file);
 
-    FileType GetFileType() override {
+    FileType GetFileType() const override {
         return IdentifyType(file);
     }