Fix `unable to find library -lrt` on OpenBSD.
Fix `unable to find library -lrt` on OpenBSD.

This change mirrors what is already done elsewhere in the same file for omitting `-lrt` on OpenBSD.

Attempting to include `-lrt` results in the following error on OpenBSD:

```
ERROR: /home/ad/tree/src/main/tools/BUILD:28:10: Linking src/main/tools/process-wrapper failed: (Exit 1): clang failed: error executing command (from target //src/main/tools:process-wrapper)
  (cd /tmp/bazel_4ds6S24b/out/execroot/io_bazel && \
  exec env - \
    PATH=/home/ad/bin:/bin:/sbin:/usr/bin:/usr/sbin:/usr/X11R6/bin:/usr/local/bin:/usr/local/sbin:/usr/games:/usr/local/jdk-11/bin \
    PWD=/proc/self/cwd \
  /usr/bin/clang -o bazel-out/openbsd-opt/bin/src/main/tools/process-wrapper bazel-out/openbsd-opt/bin/src/main/tools/_objs/process-wrapper/process-wrapper.o bazel-out/openbsd-opt/bin/src/main/tools/_objs/process-wrapper/process-wrapper-legacy.o bazel-out/openbsd-opt/bin/src/main/tools/_objs/process-wrapper/process-wrapper-options.o bazel-out/openbsd-opt/bin/src/main/tools/libprocess-tools.a bazel-out/openbsd-opt/bin/src/main/protobuf/libexecution_statistics_proto.a bazel-out/openbsd-opt/bin/external/com_google_protobuf/libprotobuf.a bazel-out/openbsd-opt/bin/external/com_google_protobuf/libprotobuf_lite.a bazel-out/openbsd-opt/bin/third_party/zlib/libzlib_checked_in.a bazel-out/openbsd-opt/bin/src/main/tools/liblogging.a -lm -lrt -lpthread -lm -lpthread -lm -lstdc++ -Wl,-z,relro,-z,now -no-canonical-prefixes -Wl,--gc-sections)
# Configuration: e195de8252629d94d6dacc8fcb187a6351c13c38490dddec78ec59d1c01f00f9
# Execution platform: //:default_host_platform
ld: error: unable to find library -lrt
```
PiperOrigin-RevId: 478005927
Change-Id: I7e37141d017d4128a7f37112c222b4194007af97

cushon/bazel,cushon/bazel,bazelbuild/bazel,katre/bazel,katre/bazel,katre/bazel,cushon/bazel,katre/bazel,bazelbuild/bazel,bazelbuild/bazel,bazelbuild/bazel,cushon/bazel,katre/bazel,cushon/bazel,cushon/bazel,bazelbuild/bazel,bazelbuild/bazel,katre/bazel