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/llmodel_shared.h

@@ -1,9 +1,11 @@
 #pragma once
-#include <cstdint>
-#include <cstddef>
-#include <vector>
+
 #include <ggml.h>
 
+#include <cstddef>
+#include <cstdint>
+#include <vector>
+
 struct llm_buffer {
     uint8_t * addr = NULL;
     size_t size = 0;