apparmor: move the "conflicting profile attachments" infostr to a const declaration

apparmor: move the "conflicting profile attachments" infostr to a const declaration

Instead of having a literal, making this a constant will allow for (hacky)
detection of conflicting profile attachments from inspection of the info
pointer. This is used in the next patch to augment the information provided
through domain.c:x_to_label for ix/ux fallback.

Signed-off-by: Ryan Lee <ryan.lee@canonical.com>
Signed-off-by: John Johansen <john.johansen@canonical.com>