ustr: not available for static-only configurations

ustr now unconditionally builds a shared library, so we have to mark
it as not available for static-only builds. The only reverse
dependencies of ustr is libsemanage, which was anyway already not
available for pure static builds.

Fixes:

  http://autobuild.buildroot.org/results/734/734975f6ac041e8aece07205c0b8966fe51e7df1/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>