fix external .pxd overrides for final classes

Cython/Compiler/Symtab.py

@@ -1830,7 +1830,7 @@ class CClassScope(ClassScope):
                 if defining and entry.func_cname:
                     error(pos, "'%s' already defined" % name)
                 #print "CClassScope.declare_cfunction: checking signature" ###
-                if entry.is_final_cmethod:
+                if entry.is_final_cmethod and entry.is_inherited:
                     error(pos, "Overriding final methods is not allowed")
                 elif type.same_c_signature_as(entry.type, as_cmethod = 1) and type.nogil == entry.type.nogil:
                     pass