uml: remove left-overs from previous kernel bumps

uml: remove left-overs from previous kernel bumps

This should have been fixed by an upstream patch in 5.12
|commit ed102bf2afed226703eaf85a704755bdbea34583
|Author: Randy Dunlap <[email protected]>
|Date:   Thu Apr 15 10:13:52 2021 -0700
|
|    um: Fix W=1 missing-include-dirs warnings
|
|    Currently when using "W=1" with UML builds, there are over 700 warnings
|    cc1: warning: ./arch/um/include/uapi: No such file or directory [-Wmissing-include-dirs]
|
|    but arch/um/ does not have include/uapi/ at all, so add that
|    subdir and put one Kbuild file into it (since git does not track
|    empty subdirs).

Signed-off-by: Christian Lamparter <[email protected]>