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/nax.cpp

@@ -37,7 +37,7 @@ FileType AppLoader_NAX::IdentifyType(const FileSys::VirtualFile& file) {
     return IdentifyTypeImpl(nax);
 }
 
-FileType AppLoader_NAX::GetFileType() {
+FileType AppLoader_NAX::GetFileType() const {
     return IdentifyTypeImpl(*nax);
 }