uclibc-ng-test: pass LDFLAGS while building

When static linking is in use we need to pass -static while
linking.

Fixes:
  http://autobuild.buildroot.net/results/37ca35854a36fab1690f6ae789e338a1e55307f6



Signed-off-by: Waldemar Brodkorb <wbx@openadk.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>