build.CustomTarget: Also accept mesonlib.File types as 'command' arguments

mesonbuild/build.py

@@ -808,6 +808,8 @@ class CustomTarget:
                     if not isinstance(s, str):
                         raise InvalidArguments('Array as argument %d contains a non-string.' % i)
                     final_cmd.append(s)
+            elif isinstance(c, File):
+                final_cmd.append(os.path.join(c.subdir, c.fname))
             else:
                 raise InvalidArguments('Argument %s in "command" is invalid.' % i)
         self.command = final_cmd