lvm2: disable static builds

When BR2_PREFER_STATIC_LIB is defined it still try to build the shared
executables and it fails.

Fixes:
http://autobuild.buildroot.net/results/b0b/b0b386969459dda9c294f1ccb4927ca225fa6bdd/



Signed-off-by: Fabio Porcedda <fabio.porcedda@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>