SGPath: make read_allowed_paths and write_allowed_paths static data members

No functional change, only a tidier organization.

simgear/misc/sg_path.cxx

@@ -53,9 +53,10 @@
 using std::string;
 using simgear::strutils::starts_with;
 
-// For SGPath::validate()
-static string_list read_allowed_paths;
-static string_list write_allowed_paths;
+// Static data members used by SGPath::addAllowedPathPattern(),
+// SGPath::validate(), etc.
+string_list SGPath::read_allowed_paths;
+string_list SGPath::write_allowed_paths;
 
 /**
  * define directory path separators

simgear/misc/sg_path.hxx

@@ -396,6 +396,10 @@ private:
     mutable bool _existsCached : 1; ///< only used on Windows
     mutable time_t _modTime;
     mutable size_t _size;
+
+    // For addAllowedPathPattern(), validate(), etc.
+    static string_list read_allowed_paths;
+    static string_list write_allowed_paths;
 };
 
 // Other comparison operators are in the class definition block