nax: Avoid re-parsing NAX data with GetFileType()

An instance of the NAX apploader already has an existing NAX instance in
memory. Calling directly into IdentifyType() directly would re-parse the
whole file again into yet another NAX instance, only to toss it away
again.

This gets rid of unnecessary/redundant file parsing and allocations.

src/core/loader/nax.h

@@ -31,9 +31,7 @@ public:
      */
     static FileType IdentifyType(const FileSys::VirtualFile& file);
 
-    FileType GetFileType() override {
-        return IdentifyType(file);
-    }
+    FileType GetFileType() override;
 
     ResultStatus Load(Kernel::SharedPtr<Kernel::Process>& process) override;