[PATCH v2] [gdb] Fix compile error due to [[noreturn]] with clang

[Tom Tromey]

>>>>> andrew  <andrew@andrewoates.com> writes:

> -[[noreturn]] static void gdb_os_error (host_callback *, const char *, ...);
> +static void gdb_os_error(host_callback *, const char *, ...)
> +  ATTRIBUTE_NORETURN;
 
I fixed the little formatting error here (removed a space) and added my
approved-by.  I'm going to push it in a second.

Tom