Fortran: Fix regression caused by first patch [PR125527]

2026-06-07  Paul Thomas  <pault@gcc.gnu.org>

gcc/fortran
	PR fortran/125527
	* class.cc (gfc_find_derived_vtab): Stash module namespace and
	use it as the last place to search for the vtab before building
	a new one.

(cherry picked from commit 62b2b65c59d3f26e090ae74879df379163fda407)