mjpg-streamer: disable for static builds

This package needs to load plugins to do its job. Those plugins are
shared objects (.so) which are loaded using 'dlopen()', so it makes no
sense to enable this package when doing static builds where 'dlopen()'
is not available.

Fixes:

  http://autobuild.buildroot.net/results/cd5/cd52b739370d57b5ecbc6472b8c7f1126700e85f/



Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>