m4: Don't suggest attribute malloc

m4: Don't suggest attribute malloc

With glib inclusion, some of its functions have
__attribute__((__malloc__)) which make compiler realize we want
to use the same attribute for some trivial functions of ours. For
instance qemuDomainManagedSavePath(). I don't see any real
benefit into using the attribute, so disable that suggestion.

In fact, wrong use of the attribute may lead to mysterious bugs:

  https://gitlab.gnome.org/GNOME/glib/issues/1465

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
ACKed-by: Peter Krempa <pkrempa@redhat.com>