tests/etnaviv: link against libdrm

Fixes link errors detected by buildroot autobuilders:

http://autobuild.buildroot.net/results/68a/68af62f4ab6944d326468818562c05fd5cc55b03/build-end.log

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=100530


Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>