libebml: properly support static only builds

This commit adjusts the libebml package to not build a shared library
when BR2_PREFER_STATIC_LIB=y.

Fixes:

  http://autobuild.buildroot.org/results/262/26279f72b6c5708f363b3cce2102afa0eccc3a72/



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