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.h

@@ -2,14 +2,15 @@
 #define LLMODEL_H
 
 #include <algorithm>
+#include <cstddef>
 #include <cstdint>
-#include <fstream>
 #include <functional>
-#include <limits>
 #include <optional>
+#include <stdexcept>
 #include <string>
 #include <string_view>
 #include <unordered_map>
+#include <utility>
 #include <vector>
 
 using namespace std::string_literals;