How to avoid a lot of './arch/um/include/uapi: No such file or directory' warnings

root@ubuntu18:~/linux-next# make W=1 ARCH=um allmodconfig
...
root@ubuntu18:~/linux-next# make W=1 ARCH=um prepare
...
  UPD     include/generated/asm-offsets.h
  CALL    scripts/checksyscalls.sh
cc1: warning: ./arch/um/include/uapi: No such file or directory
[-Wmissing-include-dirs]
  CALL    scripts/atomic/check-atomics.sh
...
root@ubuntu18:~/linux-next# make W=1 ARCH=um modules_prepare
...
  CC      scripts/mod/empty.o
cc1: warning: ./arch/um/include/uapi: No such file or directory
[-Wmissing-include-dirs]
  MKELF   scripts/mod/elfconfig.h
  HOSTCC  scripts/mod/modpost.o
  CC      scripts/mod/devicetable-offsets.s
cc1: warning: ./arch/um/include/uapi: No such file or directory
[-Wmissing-include-dirs]
  HOSTCC  scripts/mod/file2alias.o
  HOSTCC  scripts/mod/sumversion.o
  HOSTLD  scripts/mod/modpost
  CC      kernel/bounds.s
cc1: warning: ./arch/um/include/uapi: No such file or directory
[-Wmissing-include-dirs]
  CC      arch/um/kernel/asm-offsets.s
cc1: warning: ./arch/um/include/uapi: No such file or directory
[-Wmissing-include-dirs]
In file included from ./include/linux/uaccess.h:11:0,
                 from ./include/linux/crypto.h:21,
                 from ./arch/x86/um/shared/sysdep/kernel-offsets.h:5,
                 from arch/um/kernel/asm-offsets.c:1:
./arch/um/include/asm/uaccess.h: In function ‘__access_ok’:
./arch/um/include/asm/uaccess.h:17:29: warning: comparison of unsigned
expression >= 0 is always true [-Wtype-limits]
    (((unsigned long) (addr) >= FIXADDR_USER_START) && \
                             ^
./arch/um/include/asm/uaccess.h:45:3: note: in expansion of macro
‘__access_ok_vsyscall’
   __access_ok_vsyscall(addr, size) ||
   ^~~~~~~~~~~~~~~~~~~~
In file included from arch/um/kernel/asm-offsets.c:1:0:
./arch/x86/um/shared/sysdep/kernel-offsets.h: At top level:
./arch/x86/um/shared/sysdep/kernel-offsets.h:9:6: warning: no previous
prototype for ‘foo’ [-Wmissing-prototypes]
 void foo(void)
      ^~~
  CALL    scripts/checksyscalls.sh
cc1: warning: ./arch/um/include/uapi: No such file or directory
[-Wmissing-include-dirs]
...
root@ubuntu18:~/linux-next# make W=1 ARCH=um
...
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