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

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

    HDRTEST usr/include/asm/stat.h
  In file included from <command-line>:32:
  ./usr/include/asm/stat.h:22:2: error: unknown type name 'ino_t'
     22 |  ino_t  st_ino;
        |  ^~~~~
  ./usr/include/asm/stat.h:23:2: error: unknown type name 'mode_t'
     23 |  mode_t  st_mode;
        |  ^~~~~~
  ./usr/include/asm/stat.h:25:2: error: unknown type name 'uid_t'
     25 |  uid_t  st_uid;
        |  ^~~~~
  ./usr/include/asm/stat.h:26:2: error: unknown type name 'gid_t'
     26 |  gid_t  st_gid;
        |  ^~~~~
  ./usr/include/asm/stat.h:58:2: error: unknown type name 'mode_t'
     58 |  mode_t  st_mode;
        |  ^~~~~~
  ./usr/include/asm/stat.h:61:2: error: unknown type name 'uid_t'
     61 |  uid_t  st_uid;
        |  ^~~~~
  ./usr/include/asm/stat.h:62:2: error: unknown type name 'gid_t'
     62 |  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>