colo: use local path for local headers

colo: use local path for local headers

When pulling in headers that are in the same directory as the C file (as
opposed to one in include/), we should use its relative path, without a
directory.

Reviewed-by: Zhang Chen <zhangckid@gmail.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>