Move header outside Unix-only portion

We'll neeed some definitions elsewhere

src/libstore/include/nix/store/build/derivation-builder.hh

@@ -188,7 +188,9 @@ struct DerivationBuilder : RestrictionContext
     virtual void killSandbox(bool getStats) = 0;
 };
 
+#ifndef _WIN32 // TODO enable `DerivationBuilder` on Windows
 std::unique_ptr<DerivationBuilder> makeDerivationBuilder(
     LocalStore & store, std::unique_ptr<DerivationBuilderCallbacks> miscMethods, DerivationBuilderParams params);
+#endif
 
 } // namespace nix

src/libstore/include/nix/store/meson.build

@@ -12,6 +12,7 @@ config_pub_h = configure_file(
 headers = [ config_pub_h ] + files(
   'binary-cache-store.hh',
   'build-result.hh',
+  'build/derivation-builder.hh',
   'build/derivation-building-goal.hh',
   'build/derivation-building-misc.hh',
   'build/derivation-goal.hh',

src/libstore/unix/include/nix/store/meson.build

@@ -2,7 +2,6 @@ include_dirs += include_directories('../..')
 
 headers += files(
   'build/child.hh',
-  'build/derivation-builder.hh',
   'build/hook-instance.hh',
   'user-lock.hh',
 )