[PATCH xserver] tests: link against libos too

[Mihail Konev]

Thanks and sorry for late response.
With dtrace being present, this breaks the build.
Conditioned patch attached.
-------------- next part --------------
A non-text attachment was scrubbed...
Name: 0001-tests-Fix-without-dtrace-build.patch
Type: text/x-diff
Size: 825 bytes
Desc: not available
URL: <https://lists.x.org/archives/xorg-devel/attachments/20170114/8d81b25c/attachment.patch>