ninjabackend: use File.from_built_relative()

Do not reinvent it in NinjaBackend.determine_ext_objs(), so as to use
the recently added caching of the results of File.from_built_relative().

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Eli Schwartz <eschwartz93@gmail.com>

Author: bonzini

mesonbuild/backend/backends.py

@@ -883,8 +883,7 @@ def _determine_ext_objs(self, extobj: 'build.ExtractedObjects', proj_dir_to_buil
         for gensrc in extobj.genlist:
             for r in gensrc.get_outputs():
                 path = self.get_target_generated_dir(extobj.target, gensrc, r)
-                dirpart, fnamepart = os.path.split(path)
-                raw_sources.append(File(True, dirpart, fnamepart))
+                raw_sources.append(File.from_built_relative(path))
 
         # Filter out headers and all non-source files
         sources: T.List['FileOrString'] = []