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/utils.cpp

@@ -1,7 +1,12 @@
 #include "utils.h"
 
+#include <cmath>
+#include <cstdio>
+#include <cstdlib>
 #include <fstream>
+#include <iterator>
 #include <regex>
+#include <utility>
 
 void replace(std::string & str, const std::string & needle, const std::string & replacement) {
     size_t pos = 0;
@@ -325,4 +330,4 @@ gpt_vocab::id gpt_sample_top_k_top_p(
     int idx = dist(rng);
 
     return logits_id[idx].second;
-}
+}