Add an extra suppression.

On Fedora 33 with gcc (GCC) 10.2.1 20201125 (Red Hat 10.2.1-9)
it looks like fun:__static_initialization_and_destruction_0 is
now inlined which causes the existing suppression for the
same reachable to no longer match.

memcheck/tests/libstdc++.supp

@@ -96,3 +96,15 @@
    fun:_dl_init
    fun:_dl_start_user
 }
+{
+   malloc-leaks-cxx-stl-string-classes-debug3
+   Memcheck:Leak
+   match-leak-kinds: reachable
+   fun:malloc
+   fun:pool
+   fun:UnknownInlinedFun
+   fun:_GLOBAL__sub_I_eh_alloc.cc
+   fun:call_init.part.0
+   fun:_dl_init
+   obj:*lib*/ld-2.*.so
+}