Linking mimalloc-test-stress-dynamic fails on OpenBSD

[hiddener]

Commit 0774345 breaks mimalloc-test-stress-dynamic linking:

[100%] Linking C executable mimalloc-test-stress-dynamic
ld: warning: test-stress.c(CMakeFiles/mimalloc-test-stress-dynamic.dir/test/test-stress.c.o:(main)): warning: rand() may return deterministic values, is that what you want?
ld: error: undefined symbol: pthread_create
>>> referenced by test-stress.c
>>>               CMakeFiles/mimalloc-test-stress-dynamic.dir/test/test-stress.c.o:(main)

ld: error: undefined symbol: pthread_join
>>> referenced by test-stress.c
>>>               CMakeFiles/mimalloc-test-stress-dynamic.dir/test/test-stress.c.o:(main)
cc: error: linker command failed with exit code 1 (use -v to see invocation)
*** Error 1 in . (CMakeFiles/mimalloc-test-stress-dynamic.dir/build.make:101 'mimalloc-test-stress-dynamic': /usr/local/bin/cmake -E cmake_l...)
*** Error 2 in . (CMakeFiles/Makefile2:332 'CMakeFiles/mimalloc-test-stress-dynamic.dir/all': make -s -f CMakeFiles/mimalloc-test-stress-dyn...)
*** Error 2 in /home/test/mimalloc/out/release (Makefile:146 'all': make -s -f CMakeFiles/Makefile2 all)