ninjabackend: add missing type annotation

I needed to figure this out for the purposes of annotating CleanTrees
anyway.

mesonbuild/backend/ninjabackend.py

@@ -3086,7 +3086,7 @@ https://gcc.gnu.org/bugzilla/show_bug.cgi?id=47485'''))
             except OSError:
                 mlog.debug("Library versioning disabled because we do not have symlink creation privileges.")
 
-    def generate_custom_target_clean(self, trees):
+    def generate_custom_target_clean(self, trees: T.List[str]) -> str:
         e = NinjaBuildElement(self.all_outputs, 'meson-clean-ctlist', 'CUSTOM_COMMAND', 'PHONY')
         d = CleanTrees(self.environment.get_build_dir(), trees)
         d_file = os.path.join(self.environment.get_scratch_dir(), 'cleantrees.dat')