libmatroska: properly support static-only build

This is similar to commit 9cef35b6 on
libebml, since libmatroska uses the same Makefile logic as libebml. It
makes sure BR2_PREFER_STATIC_LIB=y succeed by not building the shared
library in this case.

Fixes:

  http://autobuild.buildroot.org/results/12e/12e752330b30adb5078acd72a548cc172fa9ecb8/



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