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/loader.h

@@ -131,7 +131,7 @@ public:
      * Returns the type of this file
      * @return FileType corresponding to the loaded file
      */
-    virtual FileType GetFileType() = 0;
+    virtual FileType GetFileType() const = 0;
 
     /**
      * Load the application and return the created Process instance