Fix: Build failure without OpenVDB

Caused by 0837037d13.

source/blender/blenkernel/BKE_volume_grid_fields.hh

@@ -6,7 +6,9 @@
 
 #include "FN_field.hh"
 
-#include "openvdb_fwd.hh"
+#ifdef WITH_OPENVDB
+
+#  include "openvdb_fwd.hh"
 
 namespace blender::bke {
 
@@ -46,3 +48,5 @@ class TilesFieldContext : public fn::FieldContext {
 };
 
 }  // namespace blender::bke
+
+#endif

source/blender/blenkernel/intern/volume_grid_fields.cc

@@ -4,7 +4,10 @@
 
 #include "BKE_geometry_fields.hh"
 #include "BKE_volume_grid_fields.hh"
-#include <openvdb/math/Transform.h>
+
+#ifdef WITH_OPENVDB
+
+#  include <openvdb/math/Transform.h>
 
 namespace blender::bke {
 
@@ -71,3 +74,5 @@ GVArray TilesFieldContext::get_varray_for_input(const fn::FieldInput &field_inpu
 }
 
 }  // namespace blender::bke
+
+#endif

source/blender/nodes/intern/volume_grid_function_eval.cc

@@ -14,14 +14,21 @@
 #include "BKE_volume_openvdb.hh"
 
 #include <fmt/format.h>
-#include <openvdb/Grid.h>
-#include <openvdb/math/Transform.h>
-#include <openvdb/tools/Merge.h>
+
+#ifdef WITH_OPENVDB
+
+#  include <openvdb/Grid.h>
+#  include <openvdb/math/Transform.h>
+#  include <openvdb/tools/Merge.h>
+
+#endif
 
 #include "volume_grid_function_eval.hh"
 
 namespace blender::nodes {
 
+#ifdef WITH_OPENVDB
+
 template<typename GridT>
 static constexpr bool is_supported_grid_type = is_same_any_v<GridT,
                                                              openvdb::FloatGrid,
@@ -710,4 +717,18 @@ bool execute_multi_function_on_value_variant__volume_grid(
   return true;
 }
 
+#else
+
+bool execute_multi_function_on_value_variant__volume_grid(
+    const mf::MultiFunction & /*fn*/,
+    const Span<bke::SocketValueVariant *> /*input_values*/,
+    const Span<bke::SocketValueVariant *> /*output_values*/,
+    std::string &r_error_message)
+{
+  r_error_message = TIP_("Compiled without OpenVDB");
+  return false;
+}
+
+#endif
+
 }  // namespace blender::nodes