build: fix type annotation issue

I noticed by inspection

mesonbuild/build.py

@@ -426,7 +426,7 @@ class ExtractedObjects(HoldableObject):
         # Filter out headers and all non-source files
         return [s for s in sources if environment.is_source(s) and not environment.is_header(s)]
 
-    def classify_all_sources(self, sources: T.List[str], generated_sources: T.Sequence['GeneratedTypes']) -> T.Dict['Compiler', T.List['FileOrString']]:
+    def classify_all_sources(self, sources: T.List[FileOrString], generated_sources: T.Sequence['GeneratedTypes']) -> T.Dict['Compiler', T.List['FileOrString']]:
         sources_ = self.get_sources(sources, generated_sources)
         return classify_unity_sources(self.target.compilers.values(), sources_)