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>