um: Fix W=1 missing-include-dirs warnings

Currently when using "W=1" with UML builds, there are over 700 warnings
like so:

CC arch/um/drivers/stderr_console.o
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: Randy Dunlap <[e-mail]>
Cc: Masahiro Yamada <[e-mail]>
Cc: Michal Marek <[e-mail]>
Cc: [e-mail]
Cc: Jeff Dike <[e-mail]>
Cc: Richard Weinberger <[e-mail]>
Cc: Anton Ivanov <[e-mail]>
Cc: [e-mail]
Reviewed-by: Masahiro Yamada <[e-mail]>
Signed-off-by: Richard Weinberger <[e-mail]>

arch/um/include/uapi/asm/Kbuild

···
+# SPDX-License-Identifier: GPL-2.0