Geometry Nodes: don't show error when using relative path in Import Nodes

Relative paths are supported, but the string input was showing an error.
This patch removes the error by tagging the string input as supporting relative files.
This feature was introduced recently in ab20edf469.

Pull Request: https://projects.blender.org/blender/blender/pulls/138707

source/blender/makesrna/intern/rna_node_socket.cc

@@ -1349,6 +1349,7 @@ static void rna_def_node_socket_string(BlenderRNA *brna,
   RNA_def_property_flag(prop, PROP_CONTEXT_UPDATE);
 
   if (subtype == PROP_FILEPATH) {
+    RNA_def_property_flag(prop, PROP_PATH_SUPPORTS_BLEND_RELATIVE);
     RNA_def_property_string_filepath_filter_func(prop, "rna_NodeSocketString_filepath_filter");
   }