Catch up OpenBSD extra-libraries to Clang 13 (#411)

The system upgrade necessitates this list to be extended.
A better solution should be in cabal, but for now we have
no choice: haskell/cabal#8007

Author: blackgnezdo

text.cabal

@@ -93,7 +93,7 @@ library
     if os(darwin) || os(freebsd)
       extra-libraries: c++
     elif os(openbsd)
-      extra-libraries: c++ c++abi
+      extra-libraries: c++ c++abi pthread
     else
       extra-libraries: stdc++