When building with clang, disable -Wunneeded-internal-declaration for

When building with clang, disable -Wunneeded-internal-declaration for
several sys/cam/ctl files, since these get the following warnings:

In file included from sys/cam/ctl/ctl_backend.c:60:
sys/cam/ctl/ctl_private.h:300:30: error: variable 'page_index_template' is not needed and will not be emitted [-Werror,-Wunneeded-internal-declaration]
static struct ctl_page_index page_index_template[] = {
                             ^

These warnings are tricky to fix without a lot of overhaul, and they are
harmless, so disable them for now.

MFC after:	1 week