lttng-libust: requires dynamic library support

Fixes:
http://autobuild.buildroot.net/results/e89/e8974516173ea74c830b71e400fcbafdd69dea60/
http://autobuild.buildroot.net/results/cac/cac7015c3be7e34dbeaa6accb59aebebe68eb6d2/
http://autobuild.buildroot.net/results/4ce/4ce0a35316cd25536b58b50f096b6f75f6e3c1c9/



Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>