meson: link libpng independent of vnc

Currently png support is dependent on vnc for linking object file to
libpng. This commit makes the parameter independent of vnc as it breaks
system emulator with --disable-vnc unless --disable-png is added.

Fixes: 9a0a119a ("Added parameter to take screenshot with screendump as PNG", 2022-04-27)
Signed-off-by: Kshitij Suri <kshitij.suri@nutanix.com>
Message-Id: <20220510161932.228481-1-kshitij.suri@nutanix.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>