checkasm: Readd a dependency on threads

3a2a874994, which switched to using
the checkasm core from the separate checkasm project, removed the
thread dependency from the checkasm executable, as the checkasm
library itself has a thread dependency.

However, checkasm doesn't always include that thread dependency,
it only does that when pthread_setaffinity_np is detected.

The dav1d object files themselves use pthreads as well, causing
undefined symbols if checkasm doesn't link in pthreads.

This should fix linking on OpenBSD after
3a2a874994, fixing issue #467.

tests/meson.build

@@ -84,6 +84,7 @@ if is_asm_enabled and checkasm_dependency.found()
         include_directories: dav1d_inc_dirs,
         build_by_default: false,
         dependencies: [
+            thread_dependency,
             libm_dependency,
             checkasm_dependency,
             ],