arch: ipcbuf.h: make uapi asm/ipcbuf.h self-contained

Userspace cannot compile <asm/ipcbuf.h> due to some missing type
definitions.  For example, building it for x86 fails as follows:

  CC      usr/include/asm/ipcbuf.h.s
In file included from ./usr/include/asm/ipcbuf.h:1:0,
                 from <command-line>:32:
./usr/include/asm-generic/ipcbuf.h:21:2: error: unknown type name `__kernel_key_t'
  __kernel_key_t  key;
  ^~~~~~~~~~~~~~
./usr/include/asm-generic/ipcbuf.h:22:2: error: unknown type name `__kernel_uid32_t'
  __kernel_uid32_t uid;
  ^~~~~~~~~~~~~~~~
./usr/include/asm-generic/ipcbuf.h:23:2: error: unknown type name `__kernel_gid32_t'
  __kernel_gid32_t gid;
  ^~~~~~~~~~~~~~~~
./usr/include/asm-generic/ipcbuf.h:24:2: error: unknown type name `__kernel_uid32_t'
  __kernel_uid32_t cuid;
  ^~~~~~~~~~~~~~~~
./usr/include/asm-generic/ipcbuf.h:25:2: error: unknown type name `__kernel_gid32_t'
  __kernel_gid32_t cgid;
  ^~~~~~~~~~~~~~~~
./usr/include/asm-generic/ipcbuf.h:26:2: error: unknown type name `__kernel_mode_t'
  __kernel_mode_t  mode;
  ^~~~~~~~~~~~~~~
./usr/include/asm-generic/ipcbuf.h:28:35: error: `__kernel_mode_t' undeclared here (not in a function)
  unsigned char  __pad1[4 - sizeof(__kernel_mode_t)];
                                   ^~~~~~~~~~~~~~~
./usr/include/asm-generic/ipcbuf.h:31:2: error: unknown type name `__kernel_ulong_t'
  __kernel_ulong_t __unused1;
  ^~~~~~~~~~~~~~~~
./usr/include/asm-generic/ipcbuf.h:32:2: error: unknown type name `__kernel_ulong_t'
  __kernel_ulong_t __unused2;
  ^~~~~~~~~~~~~~~~

It is just a matter of missing include directive.

Include <linux/posix_types.h> to make it self-contained, and add it to
the compile-test coverage.

Link: http://lkml.kernel.org/r/20191030063855.9989-1-yamada.masahiro@socionext.com


Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>