backend: fix #includes with include-what-you-use (#2371)

Also fix a PARENT_SCOPE warning when building the backend.

Signed-off-by: Jared Van Bortel <jared@nomic.ai>

gpt4all-backend/llamamodel_impl.h

@@ -4,11 +4,12 @@
 #ifndef LLAMAMODEL_H
 #define LLAMAMODEL_H
 
+#include "llmodel.h"
+
 #include <functional>
 #include <memory>
 #include <string>
 #include <vector>
-#include "llmodel.h"
 
 struct LLamaPrivate;
 struct EmbModelSpec;