[ob] user-regs.c build fix for arm-linux

[Andrew Cagney]

> +/* This structure is named gdb_user_regs instead of user_regs to avoid
> +   conflicts with any "struct user_regs" in system headers.  For instance,
> +   on ARM GNU/Linux native builds, nm-linux.h includes <signal.h> includes
> +   <sys/ucontext.h> includes <sys/procfs.h> includes <sys/user.h>, which
> +   declares "struc user_regs".  */

Even shorter, for instance:

/* This structure is named gdb_user_regs, instead of user_regs, to avoid 
  a comflict with <sys/user.h> defined "struct user_regs" on ARM 
GNU/Linux  (included by: nm-linux.h -> <signal.h> -> <sys/ucontext.h> -> 
<sys/procfs.h>).  */

which is just stating the facts.

Andrew