collectd: fix build failure with libnotify in sysroot

If libnotify is in the sysroot, collectd will try to use it and then
fail at do_compile because gtk/gtk.h can't be found. Since this is
functionality that's not likely to be needed in the embedded context as
far as collectd goes, just disable it.

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>