xen/trace: Fix build with !CONFIG_TRACEBUFFER

xen/trace: Fix build with !CONFIG_TRACEBUFFER

GCC reports:

In file included from hvm.c:24:0:
/local/xen.git/xen/include/xen/trace.h: In function ‘tb_control’:
/local/xen.git/xen/include/xen/trace.h:60:13: error: ‘ENOSYS’
undeclared (first use in this function)
     return -ENOSYS;
             ^~~~~~

Include xen/errno.h to resolve the issue.  While tweaking this, add comments
to the #else and #endif, as they are a fair distance apart.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>