util: fix futex.c build on OpenBSD
When the futex code moved it removed an include which broke the build on OpenBSD.
Fixes: 095dfc6c ("util: Move the implementation of futex_wake and futex_wait from futex.h to futex.c")
When the futex code moved it removed an include which broke the build on OpenBSD.
Fixes: 095dfc6c ("util: Move the implementation of futex_wake and futex_wait from futex.h to futex.c")