package/libwebsockets: fix static link

- disable shared object build when BR2_PREFER_STATIC_LIB is set

Fixes:
  http://autobuild.buildroot.net/results/0a9/0a9f0c3c101550e73f7100f2b88a88c1f2bbad95/



Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>