nix_api: fix missing includes in headers

Forward declaration doesn't work here, since we define classes that
contain the objects

src/libexpr/c/nix_api_expr_internal.h

@@ -1,11 +1,8 @@
 #ifndef NIX_API_EXPR_INTERNAL_H
 #define NIX_API_EXPR_INTERNAL_H
 
-// forward declaration
-namespace nix {
-class EvalState;
-class BindingsBuilder;
-}; // namespace nix
+#include "eval.hh"
+#include "attr-set.hh"
 
 struct State {
   nix::EvalState state;

src/libutil/c/nix_api_util_internal.h

@@ -2,11 +2,10 @@
 #define NIX_API_UTIL_INTERNAL_H
 
 #include <string>
+#include <optional>
 
-// forward declaration
-namespace nix {
-class Error;
-};
+#include "error.hh"
+#include "nix_api_util.h"
 
 struct nix_c_context {
   nix_err last_err_code = NIX_OK;