build: fix annotations for BuildTarget.link*target

which do not hold BuildTargets, they are `SharedLibrary | StaticLibrary |
CustomTarget | CustomTargetIndex` (whole doesn't accept `SharedLIbrary`)

mesonbuild/build.py

@@ -749,8 +749,8 @@ class BuildTarget(Target):
         self.external_deps: T.List[dependencies.Dependency] = []
         self.include_dirs: T.List['IncludeDirs'] = []
         self.link_language = kwargs.get('link_language')
-        self.link_targets: T.List[T.Union['BuildTarget', 'CustomTarget', 'CustomTargetIndex']] = []
-        self.link_whole_targets = []
+        self.link_targets: T.List[T.Union[SharedLibrary, StaticLibrary, 'CustomTarget', 'CustomTargetIndex']] = []
+        self.link_whole_targets: T.List[T.Union[StaticLibrary, CustomTarget, CustomTargetIndex]] = []
         self.link_depends = []
         self.added_deps = set()
         self.name_prefix_set = False