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: Linus Torvalds <torvalds@linux-foundation.org>

usr/include/Makefile

@@ -16,7 +16,6 @@ override c_flags = $(UAPI_CFLAGS) -Wp,-MD,$(depfile) -I$(objtree)/usr/include
 # Please consider to fix the header first.
 #
 # Sorted alphabetically.
-header-test- += asm/ipcbuf.h
 header-test- += asm/msgbuf.h
 header-test- += asm/sembuf.h
 header-test- += asm/shmbuf.h