powerpc: add asm/stat.h to UAPI compile-test coverage

asm/stat.h is currently excluded from the UAPI compile-test for
ARCH=powerpc because of the errors like follows:

    HDRTEST usr/include/asm/stat.h
  In file included from <command-line>:32:
  ./usr/include/asm/stat.h:32:2: error: unknown type name 'ino_t'
     32 |  ino_t  st_ino;
        |  ^~~~~
  ./usr/include/asm/stat.h:35:2: error: unknown type name 'mode_t'
     35 |  mode_t  st_mode;
        |  ^~~~~~
  ./usr/include/asm/stat.h:40:2: error: unknown type name 'uid_t'
     40 |  uid_t  st_uid;
        |  ^~~~~
  ./usr/include/asm/stat.h:41:2: error: unknown type name 'gid_t'
     41 |  gid_t  st_gid;
        |  ^~~~~

The errors can be fixed by prefixing the types with __kernel_.

Then, remove the no-header-test entry from user/include/Makefile.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Christoph Hellwig <hch@lst.de>