package/fbv: Fix static linking against libpng

Fixes
http://autobuild.buildroot.net/results/9ca/9cacec257f979745a7d4a43224b243ee4e16f9ac/
http://autobuild.buildroot.net/results/996/996e42396546555e9fcf0c7135aac610ccca4eee/
http://autobuild.buildroot.net/results/c91/c918046900045c34c73e16578026cce3e6bcccf8/



[Peter: adjust description, pass --static to pkg-config, only use for static linking]
Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>