Copy missed fix for OpenBSD over from src/chrono.cpp

Index: libcxx/src/filesystem/filesystem_clock.cpp
--- libcxx/src/filesystem/filesystem_clock.cpp.orig
+++ libcxx/src/filesystem/filesystem_clock.cpp
@@ -36,8 +36,10 @@
 #  define _LIBCPP_HAS_TIMESPEC_GET
 #endif
 
-#if defined(__APPLE__) || defined(__gnu_hurd__) || defined(__AMDGPU__) || defined(__NVPTX__) ||                        \
-    (defined(_POSIX_TIMERS) && _POSIX_TIMERS > 0)
+// OpenBSD and GPU do not have a fully conformant suite of POSIX timers, but
+// it does have clock_gettime and CLOCK_MONOTONIC which is all we need.
+#if defined(__APPLE__) || defined(__gnu_hurd__) || defined(__OpenBSD__) || defined(__AMDGPU__) ||                      \
+    defined(__NVPTX__) || (defined(_POSIX_TIMERS) && _POSIX_TIMERS > 0)
 #  define _LIBCPP_HAS_CLOCK_GETTIME
 #endif
 
