linux/scc.h: make uapi linux/scc.h self-contained

Userspace cannot compile <linux/scc.h>

    CC      usr/include/linux/scc.h.s
  In file included from <command-line>:32:0:
  usr/include/linux/scc.h:20:20: error: `SIOCDEVPRIVATE' undeclared here (not in a function)
    SIOCSCCRESERVED = SIOCDEVPRIVATE,
                      ^~~~~~~~~~~~~~

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

Link: http://lkml.kernel.org/r/20191108055809.26969-1-yamada.masahiro@socionext.com


Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>