package/znc: set WANT_SWIG

package/znc: set WANT_SWIG

Set WANT_SWIG={ON,OFF} to {en,dis}able swig and avoid a random build
failures probably due to parallel build issue when extracting
pre-generated tarball:

CMake Error: Problem with archive_write_header(): Can't unlink already-existing object
CMake Error: Current file: swigpyrun.h
CMake Error: Problem extracting tar: /usr/lfs/hdd_v1/rc-buildroot-test/scripts/instance-0/output/build/znc-1.7.5/modules/modpython/generated.tar.gz

This tarball contains pre-generated files, and is not used when
host-swig is available.

Fixes:
 - http://autobuild.buildroot.org/results/f3394de616cea4f474b6d5887aa0d5f5dff05281

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>