package/qemu: disable vnc optional support for the host variant

package/qemu: disable vnc optional support for the host variant

The vnc support is enabled by default and the build system
will try to detect automatically some external libraries
installed on the host for vnc-png, vnc-jpeg and vnc-sasl.

$ ldd output/host/bin/qemu-system-aarch64
	[...]
	libpng16.so.16 => /lib64/libpng16.so.16
or
	libpng16.so.16 => output/host/lib/libpng16.so.16
if host-libpng is built before host-qemu.

Disable explicitely thoses options to improve the build
reproducibility.

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>