add RTLD_GLOBAL so loaded symbols are available to other modules

dtool/src/dtoolutil/load_dso.cxx

@@ -165,7 +165,7 @@ load_dso_error() {
 void *
 load_dso(const Filename &filename) {
   string os_specific = filename.to_os_specific();
-  return dlopen(os_specific.c_str(), RTLD_NOW);
+  return dlopen(os_specific.c_str(), RTLD_NOW | RTLD_GLOBAL);
 }
 
 bool