sourceset module: fix minor regression in making sourcesets immutable

In commit c0be7e05b0 the setting of merged
sourcesets as immutable in a loop accidentally got dedented, and only
applied to the last loop iteration.

mesonbuild/modules/sourceset.py

@@ -186,7 +186,7 @@ class SourceSetImpl(SourceSet, MutableModuleObject):
         for s in if_true:
             if not isinstance(s, SourceSetImpl):
                 raise InvalidCode('Arguments to \'add_all\' after the first must be source sets')
-        s.frozen = True
+            s.frozen = True
         self.rules.append(SourceSetRule(keys, [], [], if_true, dependencies))
 
     def collect(self, enabled_fn: T.Callable[[str], bool],