neon: disable webdav when no XML support is available

Since 9169810a ("neon: replace
'choice' for XML library with two options") the option
BR2_PACKAGE_NEON_NOXML no longer exists. However, it was not taken
into account properly within neon.mk, as this option was still used to
disable webdav support when no XML library has been selected.

Fixes:

  http://autobuild.buildroot.org/results/7ed/7ed657570d83b28389dd8c01ffdd895d2e8d145f/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>