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

asm/shmbuf.h is currently excluded from the UAPI compile-test because of
the errors like follows:

    HDRTEST usr/include/asm/shmbuf.h
  In file included from ./usr/include/asm/shmbuf.h:6,
                   from <command-line>:
  ./usr/include/asm-generic/shmbuf.h:26:33: error: field ‘shm_perm’ has incomplete type
     26 |         struct ipc64_perm       shm_perm;       /* operation perms */
        |                                 ^~~~~~~~
  ./usr/include/asm-generic/shmbuf.h:27:9: error: unknown type name ‘size_t’
     27 |         size_t                  shm_segsz;      /* size of segment (bytes) */
        |         ^~~~~~
  ./usr/include/asm-generic/shmbuf.h:40:9: error: unknown type name ‘__kernel_pid_t’
     40 |         __kernel_pid_t          shm_cpid;       /* pid of creator */
        |         ^~~~~~~~~~~~~~
  ./usr/include/asm-generic/shmbuf.h:41:9: error: unknown type name ‘__kernel_pid_t’
     41 |         __kernel_pid_t          shm_lpid;       /* pid of last operator */
        |         ^~~~~~~~~~~~~~

The errors can be fixed by replacing size_t with __kernel_size_t and by
including proper headers.

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

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