structured_sources: Allow build_tgt to be added as source

In some cases, there is a need to use a built library as a "source" for
another target. Specifically Java JAR might want to contain their own
native JNI libraries in order to provide a single file "executable".

mesonbuild/interpreter/interpreter.py

@@ -2367,7 +2367,7 @@ def func_structured_sources(
             self, node: mparser.BaseNode,
             args: T.Tuple[object, T.Optional[T.Dict[str, object]]],
             kwargs: 'TYPE_kwargs') -> build.StructuredSources:
-        valid_types = (str, mesonlib.File, build.GeneratedList, build.CustomTarget, build.CustomTargetIndex, build.GeneratedList)
+        valid_types = (str, mesonlib.File, build.GeneratedList, build.CustomTarget, build.CustomTargetIndex, build.GeneratedList, build.BuildTarget)
         sources: T.Dict[str, T.List[T.Union[mesonlib.File, 'build.GeneratedTypes']]] = collections.defaultdict(list)
 
         for arg in mesonlib.listify(args[0]):